Casein Amino Acids Research Articles

Influence of polyethylene glycol on the excretion and digestibility of amino acids in casein and tannic acid‐fed broilers

AbstractUsing the precision feeding technique, an experiment was conducted to evaluate the influence of tannic acid (TA) and polyethylene glycol (PEG) on the excretion of amino acids and the apparent and true digestibilities of casein protein in broilers. Seventy‐two 9‐week‐old broiler cockerels grouped in nine treatments of eight replicates were fed warm water (50 mL, control birds), casein alone (12 or 18 g) or casein (12 or 18 g) with TA solution (4.5 g per 10 mL) or/and PEG solution (2 g per 10 mL). Total excreta were collected for 48 h and freeze‐dried. The amino acid content of casein and excreta was determined by reverse phase high‐performance liquid chromatography. In the absence of TA the digestibility of casein was almost complete. TA increased the excretion of amino acids to a varying extent (P < 0.01). Although the digestibility of all essential and non‐essential amino acids was adversely affected by the presence of TA, raising the amount of casein from 12 to 18 g improved significantly (P < 0.05) the apparent and true digestibilities of all amino acids. PEG reduced significantly (P < 0.01) the excretion of amino acids and improved significantly (P < 0.01) the amino acid digestibility of casein in TA‐dosed birds. However, the improvement was greater when the lower level of casein (12 g) was fed. Thus PEG might play an important role in inactivating dietary tannins in the gastrointestinal tract of birds and improving protein digestibility and utilisation, particularly when the diet is low or intermediate in protein. Copyright © 2007 Society of Chemical Industry

Does protein intake alter the precursors for synthesis of lactose and non‐essential amino acids by the mammary glands of lactating mice?

The aims were to: 1) develop a [U-13C]glucose tracer approach to establish the pathways of and substrates used for milk lactose and casein synthesis in the mouse mammary gland and 2) determine the influence of protein intake on this partition and use for milk synthesis. In Study 1, we determined the time-course to reach isotopic plateau in milk lactose and casein during the feeding of [U-13C]glucose. Four mid-lactation mice (C57/BL) were fed a standard diet with 10% of dextrose replaced by [U-13C]glucose for 5 d with milk collected after 1, 3 and 5 d of tracer feeding. 13C-Isotopic and isotopomer plateau was attained in milk lactose (glucose, galactose) and all non-essential amino acids (NEAA) in milk casein by 3 d. In Study 2, six paired (intake and pups equal) sets of lactating mice were fed either a normal (N, 20%) or low (L, 10%) protein diet from d 2 to 15 of lactation. The last 3 d, [U-13C]glucose replaced 10% of diet dextrose. On d 15, milk and mammary tissue were collected. In Study 1 and 2, 13C-mass isotopomer distribution (MID) in lactose-derived glucose and galactose did not differ, indicating common mammary metabolic pools. In Study 2, L mothers and pups weighed less (P<0.01). In addition, 13C-MID in blood vs. casein indicated significant mammary synthesis of glutamate (N:51%, L:50%), alanine (N:32%, L:29%), and serine (N:18%, L:37%, P<0.05). This NEAA synthesis suggests additional requirements for glucose and(or) EAA.

Endogenous ileal losses of nitrogen and amino acids in pigs and piglets fed graded levels of casein

In order to determine ileal losses of nitrogen (N) and amino acids (AA) and the coefficients of apparent and true ileal digestibility (AID, TID) of N and AA from casein in piglets and pigs, two experiments were conducted. In Experiment 1, 24 piglets were used. The piglets were weaned at 17 days of age, weighing 6.4 kg and cannulated at terminal ileum. Ileal digesta was collected at 28 – 29 and 35 – 36 days of age in period 1 and 2, respectively. Feed intake was 150 and 300 g · d−1 during the first and second period. In Experiment 2, 16 castrates weighing 52.5 kg and cannulated at terminal ileum were used. The intake level of digestible energy was 2.5 times their maintenance requirement. The experiment lasted 7 days and ileal digesta was collected on day 6 – 7. Treatments consisted of four levels of N from casein: 8, 16, 24 and 32 g N · kg−1 feed, respectively. Results showed that N level did not increase N or AA ileal losses. In piglets, N and AA ileal losses were similar between periods, except for period 2, where losses per kg DMI were about 47 and 64% higher for glycine and proline, respectively (p < 0.05). When ileal losses from pigs and piglets were compared, piglets had higher (p < 0.05) ileal losses of N and AA (excepted glutamic acid and alanine). A lower (p < 0.05) AID was observed in piglets in period 2 for N, methionine, glutamic acid, glycine and proline. With exception of glycine in pigs, all values for TID of N and AA of casein were superior to 0.90. Piglets had higher TID of N, leucine, isoleucine, valine and phenylalanine. These results showed that piglets have higher ileal losses than pigs.

Ileal amino acid digestibility of some novel dietary protein sources for growing chickens

AbstractThe apparent ileal digestibility coefficients of amino acids in five high‐protein ingredients (casein, isolated soyprotein, maize gluten meal, spray‐dried bovine plasma (SBP) and spray‐dried porcine plasma (SPP)) and soybean meal were determined using 3‐week‐old broiler chickens. The amino acids in casein were highly digestible. Isolated soyprotein was also well digested by the birds, but the digestibility coefficients of all amino acids in isolated soyprotein were lower (P &lt; 0.05) than those in casein. The amino acid digestibility coefficients in isolated soyprotein were greater (P &lt; 0.05) than those in soybean meal, which may be reflective of reduced concentrations of anti‐nutritive factors, particularly indigestible oligosaccharides. The digestibilities of amino acids in maize gluten meal were higher (P &lt; 0.05) than those in soybean meal, except for the digestibilities of arginine, histidine, threonine and aspartic acid, which were similar (P &gt; 0.05) between the two protein sources. The ileal digestibilities of all indispensable amino acids in SBP and SPP were lower (P &lt; 0.05) than those in casein and isolated soybean meal but higher (P &lt; 0.05) than those in soybean meal. No differences (P &gt; 0.05) were observed in the digestibility of amino acids between SBP and SPP. The overall mean amino acid digestibility coefficients of casein, isolated soyprotein, soybean meal, maize gluten meal, SBP and SPP were 0.955, 0.888, 0.823, 0.849, 0.860 and 0.856 respectively. Copyright © 2006 Society of Chemical Industry

Conserved Role of the Linker α-Helix of the Bacterial Disulfide Isomerase DsbC in the Avoidance of Misoxidation by DsbB

In the bacterial periplasm the co-existence of a catalyst of disulfide bond formation (DsbA) that is maintained in an oxidized state and of a reduced enzyme that catalyzes the rearrangement of mispaired cysteine residues (DsbC) is important for the folding of proteins containing multiple disulfide bonds. The kinetic partitioning of the DsbA/DsbB and DsbC/DsbD pathways partly depends on the ability of DsbB to oxidize DsbA at rates >1000 times greater than DsbC. We show that the resistance of DsbC to oxidation by DsbB is abolished by deletions of one or more amino acids within the alpha-helix that connects the N-terminal dimerization domain with the C-terminal thioredoxin domain. As a result, mutant DsbC carrying alpha-helix deletions could catalyze disulfide bond formation and complemented the phenotypes of dsbA cells. Examination of DsbC homologues from Haemophilus influenzae, Pseudomonas aeruginosa, Erwinia chrysanthemi, Yersinia pseudotuberculosis, Vibrio cholerae (30-70% sequence identity with the Escherichia coli enzyme) revealed that the mechanism responsible for avoiding oxidation by DsbB is a general property of DsbC family enzymes. In addition we found that deletions in the linker region reduced, but did not abolish, the ability of DsbC to assist the formation of active vtPA and phytase in vivo, in a DsbD-dependent manner, revealing that interactions between DsbD and DsbC are also conserved.

Effect of low level monensin supplementation on the production of dairy cows fed alfalfa silage.

Effectiveness of low level monensin supplementation on N utilization in lactating dairy cows fed alfalfa silage was assessed using 48 multiparous Holsteins. Cows were fed a covariate diet [% of dry matter (DM): 56% alfalfa silage, 39% ground high moisture corn, 3% soybean meal, 1% ground corn, 1% vitamin-mineral supplements] for 2 wk, then grouped by days in milk into blocks of 4. Cows were randomly assigned within blocks to 1 of 4 diets that were fed for 10 wk: 1) control (covariate diet), 2) control plus 3% fish meal (replacing DM from high moisture corn), 3) monensin (10 mg/kg DM), and 4) monensin plus 3% fish meal. Diets 1 and 3 averaged 16.7% crude protein (25% from free AA in alfalfa silage); diets 2 and 4 averaged 18.5% crude protein. Monensin intake averaged 16 mg/d on diets 1 and 2 (due to contamination) and 248 mg/d on diets 3 and 4. There was no effect of fish meal or monensin on DM intake. However, weight gain and yield of milk, protein, and SNF increased with fish meal feeding, indicating metabolizable protein limited production. Feeding monensin increased blood glucose but reduced yield of 3.5% fat-corrected milk, milk fat content and yield, and milk protein content and yield. Apparent N efficiency was greatest on monensin (diet 3) but lowest on monensin plus fish meal (diet 4). Fish meal reduced blood glucose concentration and apparent N efficiency, and increased concentrations of milk and blood urea. Monensin increased ruminal propionate concentration and decreased concentration of acetate and butyrate and acetate:propionate in ruminally cannulated cows fed the experimental diets. However, these changes were small, suggesting that too little monensin was fed. Fish meal reduced ruminal total amino acid (AA) but monensin did not alter ruminal NH(3) or total AA. Both fish meal and monensin increased NH(3) formation from casein AA using ruminal inoculum from the cannulated cows. There was no evidence from this trial that feeding 250 mg of monensin per day to lactating cows improved N utilization by reducing ruminal catabolism of the large amounts of free AA in alfalfa silage.

15N enrichment of casein amino acids in the milk from goats given a single intravenous dose of L-[15N]leucine.

Branched-chain amino acids (AA) are mostly metabolized in the splanchnic area, but some are metabolized within the mammary gland and thus could contribute to the synthesis of non-essential AA (Wohlt et al. 1977). The extraction rate of leucine from plasma by the mammary gland is particularly high (64% in the goat; Roets et al. 1983), in excess of that used for the synthesis of milk proteins (Davis &amp; Mepham, 1976; Wohlt et al. 1977; Roets et al. 1983). Thus, although mammary leucine is mainly used for milk protein, it also provides amino nitrogen and carbon sources for the synthesis of non-essential AA.To our knowledge, no information is available on the transfer and distribution of plasma leucine amino nitrogen to milk protein AA. Using the technique for chromatographic fractionation of AA recently developed by Casseron et al. (1997), we studied the specific 15N enrichment of casein (CN) AA in the goat given a single intravenous injection of [15N]leucine.

Effects on growth rate and utilisation of amino acids in weaner pigs fed diets containing whole proteins and free amino acids in combination with different energy sources

This experiment was designed to determine the effects of 3 protein and 3 energy (carbohydrate) sources on the growth rate of weaner piglets and their eciency of nutrient utilisation. All 3 energy sources and 2 of the protein sources [casein plus free amino acids (casein), free amino acids alone (FAA)] were chosen because they were suitable for studies measuring amino acid utilisation. The third protein source, based predominantly on peptide protein, was a combination casein-fish meal-soyabean meal (CFS). The combinations of carbohydrates tested were sucrose alone, sucrose plus 150 g lactose/kg, and wheat starch plus 150 g sucrose plus 150 g lactose/kg. The experiment was a randomised complete block design based on a 33 factorial arrangement with 5 blocks and 9 pigs per block. Piglets given CFS as the protein source exhibited greater (P &lt; 0·001) weight gain than those consuming casein or FAA (494 v. 415 and 345 g/day) and deposited protein more eciently (P &lt; 0·001) (e.g. crude protein retention/ileal-digestible protein intake, 0·65 v. 0·56 and 0·49). The apparent digestibility of nitrogen (N) and amino acids was lower for the CFS-fed piglets (0·83 and 0·88) than for those given casein (0·92 and 0·95) or FAA (0·94 and 0·96). The daily weight gains of piglets given diets containing starch, sucrose, and lactose were 9-10% lower (P &lt; 0·05) than those consuming either sucrose or sucrose plus lactose (391 v. 433 and 430 g/day). The energy sources, however, had no effect on ileal digestibility of amino acids or empty body composition. Neither casein nor FAA diets were found to be acceptable sources of amino acids for studies on amino acid utilisation. Sucrose and sucrose plus lactose were excellent energy sources for piglets. In contrast, wheat starch was found to be unsuitable for inclusion in synthetic weaner diets for piglets with a liveweight of 5-20 kg.

15N enrichments of casein and plasma protein amino acids in cows ingesting 15N-labelled ammonium sulphate.

The aim of this work was to determine by ion-exchange liquid chromatography and isotope ratio mass spectrometry the specific 15N enrichment of amino acids in casein and plasma proteins in cows receiving three successive daily oral doses (300, 150 and 150 g) of (15NH4)2SO4 (10 atom per cent isotopic enrichment) and to examine the 15N enrichments obtained with regard to nitrogen transport and metabolism in the lactating cow. To investigate the 15N distribution in amino acids in casein and in plasma proteins, samples of 15N-labelled casein and plasma proteins were extracted either from a pool of several milkings (36-96 h after starting to administer the tracer) or from pooled venous blood (removed on the fourth day after the start of administration) from the four lactating cows. 15N enrichments of the proteins studied, expressed as atoms percent excess, were 0.2509 for casein and 0.0577 for plasma protein. Chromatographic fractionation of the amino acid mixture (protein hydrolysates) resulted in nine groups containing between one and four amino acids: Asp, Ser and Thr; Glu; Pro; Gly; Ala; Val and Met; Ileu and Leu; Tyr; Phe; His and Lys; and Arg. High 15N incorporation was demonstrated in all individual or groups of amino acids studied. In both proteins, Glu appeared to be the most enriched amino acid, Phe and Arg the least enriched. Most aliphatic molecules with a single amino group were highly enriched. The much lower (3.5-7.7-fold) enrichments in plasma protein compared with casein suggest considerable intracellular dilution at the site of liver protein synthesis. Finally, the amino acid separation methods are discussed and suggestions for improving them considered.

Effects of a Vespa amino acid mixture identical to hornet larval saliva on the blood biochemical indices of running rats

A unique amino acid mixture identical in composition to that of larval saliva of the hornet, referred to as a Vespa amino acid mixture (VAAM) was prepared; the effect of VAAM on the blood biochemical indices and plasma amino acid composition of running rats was studied. Thirty min before running exercise, VAAM, a reference amino acid mixture with a composition identical to that of bovine casein (casein amino acid mixture, CAAM), or distilled water was orally administered to the rats. The blood glucose value in the animals given VAAM did not fall after 45 min of running, the value being significantly higher than that in the rats given CAAM or water (P<0.05). There was no significant change in the blood lactate value for those rats given VAAM, whereas the value in the rats given CAAM or water had significantly increased after 45 min of running (P<0.05). The plasma NEFA value for the VAAM group was significantly higher than that for the CAAM group after 45 min of running (P<0.05). Thirty min after the administration of VAAM or CAAM, the increase in the plasma concentration of amino acids reflected the composition of the amino acid mixture. However, after 45 min of running, the concentration of total plasma amino acid in rats given VAAM was maintained at a higher level than that in the rats given CAAM (3.57±0.30 vs 2.96±0.14 μM/l); the plasma concentrations of Gly, Thr, Tyr and Pro in the rats given VAAM were significantly higher than those in the rats given CAAM. These findings suggest that some amino acids in ingested VAAM were used for gluconeogenesis or as energy sources for muscles during exercise. In addition, a promotive effect of VAAM on the utilization of body fat as an energy source could have been involved in the changes of blood biochemical indices.

Utilization of phytate and nonphytate phosphorus in chicks as affected by source and amount of vitamin D3.

Commercial and laboratory-strain crossbred chicks responded (P < .01) markedly to 1alpha-hydroxycholecalciferol (1alpha-OH D3) during the 2nd and 3rd wk of life. Bone-ash responses exceeded 50% when this compound was added at 20 microg/kg to phosphorus (P)-deficient corn-soybean meal diets containing surfeit levels (25 microg/kg) of cholecalciferol (D3). Phosphorus excretion was decreased (P < .01) and, thus, retention was increased (P < .01) when 1alpha-OH D3 was supplemented. A P-deficient (.10% P) casein-amino acid purified diet, devoid of D3, was used to determine whether 15 microg/kg of D3 was sufficient to facilitate optimal absorption of the nonphytate P contained in this diet. Bone ash responded to .075% P addition (KH2PO4), and chicks fed diets with .175% nonphytate P exhibited further bone-ash responses to 15 microg/kg of D3 or 10 microg/kg 1alpha-OH D3. Higher levels of either of these D3 compounds did not produce additional responses. This suggested that 15 to 25 microg/kg of D3 in a P-deficient corn-soybean meal diet (.28% phytate P and .14% nonphytate P) is more than adequate to facilitate optimal absorption of the nonphytate P present in the diet. A P-deficient casein-dextrose diet (.13% nonphytate P and 15 microg/kg D3) was fed in the final chick assay, and chicks fed this diet did not show bone ash responses to 1alpha-OH D3 or to microbial-derived phytase (1,470 units/kg). Thus, with P-deficient corn-soybean meal diets containing at least 15 microg D3/kg, 1alpha-OH D3 supplementation markedly increased weight gain and bone ash because it increased the utilization of phytate P.

The effect of cottonseed condensed tannins on the ileal digestibility of amino acids in casein and cottonseed kernel

The effect of adding cottonseed hulls to casein- and cottonseed-kernel-based diets on the apparent and true ileal digestibility of N and amino acids, and the proportion of this effect accounted for by condensed tannin (CT), were determined using the growing rat. Sixty rats were allocated randomly to ten semipurified diets, containing either casein (four diets) or purified unheated solvent-extracted cottonseed kernel (six diets) as the sole protein source, with Cr2O3 added as an indigestible marker. Two of the casein diets contained no hulls whilst the remaining two diets contained 70 g cottonseed hulls/kg. Two of the cottonseed-kernel-based diets contained no hulls, with two containing 23 g hulls/kg and the remaining two containing 46 g hulls/kg. For each pair of diets, PEG was either included or excluded. The effect of CT was quantified by comparing control rats (-PEG; CT acting) with PEG-supplemented rats (+PEG; CT inactivated) at each level of dietary hulls. The rats were given their respective experimental diets for 14 d. Each rat was given the food ad libitum for 10 min hourly from 08.00 to 18.00 hours. On day 14, samples of digesta were collected at death from the terminal 150 mm of ileum at 7 h from the first meal. Apparent and true ileal digestibilities were calculated for DM, N and the individual amino acids. The principal finding was that the inclusion of hulls depressed the apparent and true ileal digestibilities of N and amino acids, but with the response differing between diets. With the casein-based diet the mean apparent and true ileal amino acid digestibilities were significantly depressed from 0.89 and 0.96 to 0.85 and 0.92 respectively, by the inclusion of 70 g hulls/kg in the diet, and addition of PEG then restored these to 0.89 and 0.95. All of the depression could be explained by the CT content of the hulls. However, with the cottonseed-kernel-based diet the responses fell into three categories. The apparent and true ileal digestibilities of the essential amino acids cystine and methionine were not affected by hull addition, ileal digestibilities of leucine, isoleucine, lysine, threonine and valine were markedly depressed by hull addition with approximately 50% of the depression being explained by CT, whilst the ileal digestibilities of histidine, arginine and phenylalanine were depressed by hull addition but little or none of this effect could be explained by CT. Thus the effect of hulls on protein digestion clearly differed with source of protein. With the cottonseed-kernel-based diet it seems that components of the hulls other than CT also depressed the apparent and true ileal digestibilities of N and amino acids. The identity of these components is unknown.

Soy consumption and cholesterol reduction: review of animal and human studies.

Animal proteins such as casein are more hypercholesterolemic than soy protein or other plant proteins when fed to rabbits in low-fat, cholesterol-free, semipurified diets. A casein-amino acid mixture produces a hypercholesterolemia similar to that of casein. This appears to be mainly due to lysine and methionine, although other essential amino acids probably contribute to the effect. Arginine appeared to counteract the hypercholesterolemic effects of other essential amino acids. Soy protein gave a lower level of serum cholesterol in rabbits than did a soy protein-amino acid mixture, suggesting the presence of factors in soy protein that counteract the effects of hypercholesterolemic amino acids. Soy protein is also less hypercholesterolemic than casein in other animal species, particularly when the diet contains cholesterol, and substitution of soy protein for animal protein in the diet reduces the concentration of serum cholesterol in humans. This effect is somewhat variable but is generally greater in hypercholesterolemic than in normocholesterolemic subjects. The differing effects of dietary proteins on serum cholesterol concentrations in humans and in rabbits are primarily due to changes in LDL cholesterol, and the hypercholesterolemia produced by dietary casein is associated with down-regulation of hepatic LDL receptors.

Kinetics of blood free and milk casein-amino acid labelling in the dairy goat at two stages of lactation

The kinetics of blood free amino acids (AA) transfer into milk casein were compared in goats (n 4) at 61 (SE 5) d (Expt 1; post-peak, 4.51 (SE 0.26) kg milk/d) and at 180 (SE 6) d (Expt 2; late, 2.36 (SE 0.16) kg milk/d) of lactation during non-primed, continuous (Expt 1, 12 h; Expt 2, 16 h) intravenous infusions of mixtures of L-[1-13C]leucine and L-[1-13C]phenylalanine with either L-[1-13C]valine (Expt 1) or L-[5-13C]methionine (Expt 2). The 13C enrichments of blood free and casein-bound AA were fitted to a single exponential model to estimate isotopic plateaux and the fractional rate constant for milk casein labelling. Milk protein output and its contribution to whole-body flux was higher in Expt 1 (post-peak) than in Expt 2 (late lactation), but the kinetics of 13C labelling of the casein-bound AA were similar for all AA tracers in both experiments. At both stages of lactation the delay (6-8 h) between the attainment of isotopic plateau for the blood free AA and the corresponding attainment of plateau for the casein-bound AA indicated that the blood free pool was not the immediate precursor pool for milk casein biosynthesis. Plateau enrichments of casein-bound AA were generally higher than those for the corresponding blood free AA in both experiments. These results indicate that the relative contributions of different AA sources to the immediate precursor pool for milk casein biosynthesis are similar at different stages of lactation despite major changes in the partitioning of whole-body flux towards milk protein output. Non-milk protein fluxes were also similar in post-peak and late lactation.

Hypercholesterolemic Responses in Rabbits to Selected Groups of Dietary Essential Amino Acids

AbstractIn rabbits, elevation of LDL cholesterol is produced by feeding a cholesterol-free, semipurified diet containing 30% casein amino acid mixture, but not by feeding the same diet containing 14.7% of the casein amino acid mixture, corresponding to a normal level of dietary protein. The hypercholesterolemic response was greater when all essential amino acids except arginine or all ketogenic essential amino acids (lysine, leucine, isoleucine, threonine, phenylalanine, tryptophan and glycine) were selectively fed at three times the normal level. In the present experiments, the same high levels of lysine, leucine and methionine produced a substantial hypercholesterolemia, addition of either isoleucine + threonine or isoleucine + valine did not enhance the effect, and a mixture of threonine, histidine, phenylalanine, tryptophan and glycine gave only a moderate response. A combination of lysine and methionine produced a greater effect than either lysine and leucine or leucine and methionine. Hypercholesterolemic diets containing high levels of lysine and leucine did not cause significantly greater plasma ketone bodies or free fatty acids. Differences in growth rates and ketogenic responses were not generally correlated with hypercholesterolemia.

Cell suspension cultures from Tropaeolum majus L. establishment and growth conditions

Friable calli, obtained from Tropaeolum majus leaves, on Gamborg B5 medium supplemented with 2,4-D, kinetin and casein amino acids, were used for the establishment of cell suspension cultures in the same liquid medium. Different basal media, inoculum densities, sucrose concentrations, nitrogen sources and cysteine levels were assayed for biomass production. The best growth rates were achieved in Gamborg B5 medium supplemented with 1·5 mg litre −1 2,4-dichlorophenoxyacetic acid, 0·1 mg litre −1 kinetin, 2 g litre −1 casein amino acids, 100 mg litre −1 cysteine and with an initial inoculum of 50 g (fresh weight) litre −1. The doubling time obtained under these conditions was about 55 h.

Modulation by Casein and Soybean Protein Amino Acid Mixtures of Insulin- and Glucagon-dependent Linoleic Acid Desaturation in Primary-cultured Rat Hepatocytes

The effects of amino acid mixtures simulating to casein and soybean protein on insulin- and glucagon-dependent linoleic acid desaturation were studied in rat hepatocytes of up to 72 h of culture. The desaturation increased with the insulin molarity in the medium, but decreased with the glucagon molarity. The soybean protein type of amino acid mixture, compared to the casein type of amino acid mixture, potentiated the insulin-dependent desaturation. Thus, dietary proteins could moidify in vivo insulin-dependent intracellular metabolism through postprandial amino acids in blood plasma.

Growth, nutrition, and proteolytic activity of the sap-staining fungus Ophiostoma piceae

The sap-staining ascomycete Ophiostoma piceae strain 387N used ammonium, but not nitrate, as an inorganic nitrogen source. Organic nitrogen sources assimilated included bovine serum albumin, collagen, acid-hydrolyzed casein, urea, and various amino acids. Sucrose, glucose, maltose, raffinose, and soluble starch were suitable carbon sources. The optimum temperature for growth was near 23 °C, with an upper limit at 35 °C and minimal growth at 4 °C after 3 days. An initial pH of 6.1 yielded the greatest biomass. Proteolytic activity was greatest in cultures supplemented with protein as the nitrogen source, but some activity was detected in cultures with no assimilable source of nitrogen. Proteinases were detected throughout growth in protein-supplemented liquid media, and they appeared to hydrolyze azocoll, with optimal activity at pH 8. Isoelectric focusing gels of culture filtrates, obtained after fungal growth on protein supplemented media, showed a major proteolytic band focusing at pH 5.2. Key words: staining fungi, Ophiostoma, nutrition, biomass, proteinases.

Effect of high levels of selected dietary essential amino acids on hypercholesterolemia and down-regulation of hepatic LDL receptors in rabbits

Earlier studies showed that the elevation of serum total and low density lipoprotein (LDL) cholesterol levels produced in rabbits by feeding high levels of a casein amino acid mixture in a cholesterol-free, semipurified diet was due primarily to the essential amino acids (EAA) in the mixture. Replacing all of the non-essential amino acids in the mixture by glutamic acid (45% EAA+Glu) had little effect on the hypercholesterolemia produced by the EAA. Experiments designed to identify the hypercholesterolemic EAA showed that (i) feeding high levels of ketogenic EAA only (45% EketoAA) gave a substantial but variable elevation of serum total and LDL cholesterol and (ii) feeding high levels of all EAA except arginine (45% EAA-Arg) gave a particularly strong hypercholesterolemic response. In rabbits fed the 45% EAA-Arg diet and to a lesser extent, in those fed the 45% EAA+Glu diet, EDTA-sensitive binding of 125I-LDL to hepatic membranes in vitro was reduced compared to a control, low-cholesterolemic group fed all essential and non-essential amino acids at a level corresponding to 14.7% casein, indicating that the hypercholesterolemia was associated with down-regulation of hepatic LDL receptors.

THERMAL STABILIZATION OF FOUR MICROBIAL ?-GALACTOSIDASES BY HISTIDINE, CASEIN AMINO ACIDS AND CASEIN

Thermal stability of four microbial β-galactosidases was evaluated in the presence of casein, casein amino acids and histidine. The enzymes from Esch-erichia coli, Kluyveromyces marxianus and Streptococcus thermophilus were stabilized up to 60 fold by these additives, but there was little effect on the enzyme from Aspergillus oryzae. Stabilization by casein amino acids and histidine was largely lactose dependent. Casein amino acids were not as effective as casein for any enzyme. For the K. marxianus enzyme, histidine alone was as effective as casein and the stabilizing effect was proportional to the logarithm of the histidine concentration.