Plasma Lysine Concentrations Research Articles

Lysine-Chloride Interactions in the Growing Chick ,

Nutritional interactions between dietary chloride and lysine in the young chick were investigated. Chloride, varying over four levels from .44 to 1.84% of the diet, was included in diets containing limiting (.7%) or adequate (1.1%) levels of lysine. Increasing chloride in diets containing either level of lysine resulted in a depression of growth rate and feed consumption (P<.01). The efficiency of feed utilization for growth was significantly lowered only when chicks were fed diets containing the highest level of chloride (P<.05). Plasma lysine concentrations were greater in chicks fed the lysine adequate diet; plasma arginine was unchanged. Plasma lysine and arginine concentrations were unaffected by chloride. In a second experiment chicks were fed a lysine deficient (.7%), lysine adequate (1.2%), high lysine (2.5%), or high lysine (2.5%) high arginine (2.3%) diet with varying levels of chloride. Increasing dietary chloride depressed weight gains. This effect was most dramatic in those chicks fed the high lysine diet. Increasing dietary arginine alleviated the effect of chloride in the high lysine diet. Plasma sodium and potassium were unaffected by dietary lysine and chloride levels. Plasma chloride tended to be greater in chicks receiving high chloride diets and were highest in chicks fed the high lysine diet. We conclude that dietary chloride does not influence the lysine requirement of chicks but exacerbates the lysine-arginine antagonism.

Induction of Lysine Imbalance in Rats: Relationships between Tissue Amino Acids and Diet

We examined the ability of wheat gluten or casein diets supplemented with arginine, indispensable (IAA) or branched-chain (BCAA) amino acids (inhibitors of lysine transport into brain slices in vitro) to induce changes in tissue lysine concentrations consistent with occurrence of lysine imbalance. Tissue lysine concentrations were several-fold higher in male, 60–65 g rats of the Sprague-Dawley strain fed for 1 day a 6% casein basal diet than in rats fed wheat gluten basal diets. Feeding a casein diet containing 5% of a mixture of all IAA except lysine changed concentrations of lysine and other amino acids to resemble those from rats fed wheat gluten diets. Growth depressions caused by arginine were consistently associated with large decreases in brain lysine concentrations and large increases in tissue arginine and ornithine concentrations. Growth depressions caused by supplements of BCAA or IAA were frequently accompanied by small or insignificant decreases in brain lysine content, and by small increases in tissue content of these amino acids. Plasma lysine concentrations were generally decreased less than were those in brain. The arginine-induced decrease in brain lysine concentration is consistent with the hypothesis that induction of amino acid imbalances may involve competition for transport into the brain of an indispensable, limiting amino acid; growth depressions caused by BCAA or IAA appear to involve other factors as well.lysine imbalance lysine tissue concentrations amino acid transport

Digestion in the pig between 7 and 35 d of age. 1. The performance of pigs given milk and soya-bean proteins.

1. One hundred and fourteen pigs between 7 and 35 d of age were given diets containing milk or soya-bean proteins in four separate experiments. 2. The substitution of milk protein by soya-bean meal (SBM) to 75% of the total dietary protein reduced the weight gains and food conversion efficiencies of the pigs by 85% between 7 and 14 d of age and 31% between 21 and 28 d of age. 3. The treatment of soya-bean protein with alkali did not improve the performance of the pigs. 4. The substitution of milk protein by two different isolated soya-bean proteins (ISP) resulted in weight gains of 34 and 60% of te gains of pigs given milk protein diets. 5. Supplementation of the soya-bean-protein diets with methionine to 13 . 7 g/kg (5 g/16 g nitrogen) resulted in reduced food intakes and weight gains of the pigs and a 20-fold increase in the concentration of methionine in the blood plasma. 6. Supplementation of soya-bean-protein diets with lysine to 22 g/kg (8 g/16 g N) in addition to methionine doubled the concentration of lysine in the blood plasma. 7. The apparent digestibility (AD) of dry matter (DM) and N of the diets containing soya-bean protein increased with increasing age of the pigs, but the AD of the milk-protein diet was not affected by the age of the pigs. The retention of N as a percentage of the N intake increased from 57--67% for soya-bean protein over the duration of the experiments while the average value for the milk-fed pigs was 85% at all ages. 8. At 35 d of age, the fat content of the carcasses of the pigs given the ISP diet was 249 g/kg DM compared to 164 g/kg DM for the pigs given the milk diets.

Effect of low protein diets on free amino acids in plasma of young men: effect of wheat gluten diet.

Studies were made on alterations in plasma amino acids in young men fed a diet containing graded levels of wheat gluten. After one week on a standard diet containing 200 mgN/kg of mixed protein (animal protein content 45%), 38 young men were given a wheat gluten diet containing 170, 100, 60, 30, 15 or zero mgN/kg for 2 weeks. Blood samples measuring plasma free amino acids were taken before breakfast at the end of the periods on a standard diet and an experimental diet. In subjects on diets containing 170 to 30 mgN/kg the plasma concentrations of threonine, valine, methionine, leucine, tyrosine, phenylalanine, serine, histidine and arginine fell significantly with decrease in protein intake, but the concentration of alanine increased significantly. On the other hand, in subjects on diets containing 15 or zero mgN/kg, the plasma concentrations of the essential amino acids did not decrease, but increased to slightly more than in subjects on a diet containing 30 mgN/kg, and the alanine and glycine concentrations increased steadily. Values for plasma lysine varied from 146 +/- 22 to 194 +/- 31 mumoles/liter with gluten intakes of 170 to zero mgN/kg, but were comparable with that of 186 +/- 33 mumules/liter in subjects on a standard diet, showing that the plasma lysine concentration did not clearly reflect the dietary concentration of lysine in young men on a wheat gluten diet.

Effect of dietary lysine levels on performance, nitrogen metabolism and plasma amino acid concentrations of lactating sows

ABSTRACTDuring lactation 30 sows were given a basal diet containing 0·39% lysine or the same diet supplemented with 0·1, 0·2, 0·3 or 0·4% Llysine. In addition a group of six sows was given a high protein/high lysine control diet containing 17% crude protein and 1·06% lysine. Piglet weight gain, sow milk yield, milk contents of total solids, protein, fat and energy and efficiencies of utilization of energy and protein for milk production increased progressively as dietary lysine levels were raised to 0·59 %. Sows receiving the lysine-supplemented diets had markedly higher efficiencies of conversion of dietary protein into milk protein and lower efficiencies of energy utilization than those given the high protein/high lysine control diet.Nitrogen retention and biological value of the dietary protein increased quadratically with increasing dietary lysine concentrations, reaching maximum values at 0·59% lysine.Apparent digestibilities of isoleucine, lysine, methionine, threonine and valine in the basal diet were closely correlated with digestibility of nitrogen. Plasma lysine concentrations remained depressed and relatively constant at low dietary levels of lysine, but increased sharply at 0·59 % dietary lysine coinciding with a marked drop in circulating concentrations of other essential amino acids.

Milk Production, Nitrogen Utilization and Glucose Synthesis in Lactating Cows Infused Postruminally with Sodium Caseinate and Glucose

Four lactating Holstein cows were used in a 4 X 4 Latin Square design to determine the effects of postruminally administering sodium caseinate and/or glucose on milk production, milk composition, nitrogen utilization, amino acid utilization by the lactating mammary gland and glucose turnover rate. An 8.5% increase in milk yield and a 13.3% increase in milk protein production were obtained during infusion of sodium caseinate. No significant production responses were attributed to abomasal infusion of glucose. Arterial concentrations of most essential amino acids were increased during infusion of sodium caseinate. Uptake of phenylalanine, methionine and lysine by the mammary gland most closely paralleled their output in milk. The relative concentrations of methionine, lysine and phenylalanine in arterial plasma were considerably less than their concentrations in milk which resulted in a large percentage extraction of these amino acids by the mammary gland. If the availability of essential amino acids to the mammary gland, per se, was limiting the synthesis of milk protein, methionine, lysine and phenylalanine may have been the three amino acids most limiting. Measurements of glucose entry rate showed a trend toward increased glucose flux when either glucose, sodium caseinate or glucose plus sodium caseinate were infused abomasally. The similarity in glucose entry rates obtained during infusion of glucose and sodium caseinate suggest that the increase in milk production was not due totally to increased glucose flux resulting from sodium caseinate infusion.

The influence of abomasal supplements of zein and some amino acids on wool growth rate and plasma amino acids

SummaryMerino sheep were given abomasal infusions of (i) zein (80 g/day), (ii) zein (80 g/day) supplemented with L-lysine (6 g/day), L-tryptophan (1 g/day) or lysine and tryptophan together, or (iii) L-leucine (20 g/day). Effects on wool growth (length growth rate, fibre diameter, fibre volume growth rate and mass of wool grown) and on plasma amino acids were measured.Zein increased length growth rate and decreased fibre diameter; on average the volume growth rate and mass of wool grown were not significantly altered. The addition of tryptophan to the infusion of zein did not significantly alter wool growth, whereas the addition of lysine significantly increased all aspects of wool growth. The responses to these treatments were rapid and were adequately assessed during 12-day infusion periods. The infusion of leucine did not cause any appreciable changes in wool growth.Leucine supplementation increased the concentration of leucine in blood plasma about six-fold. Zein decreased the concentration of lysine and ornithine in plasma and increased the concentration of several essential amino acids; leucine was increased about seven-fold. The addition of tryptophan to zein had no effects on plasma amino acids, whereas lysine decreased the concentration of several amino acids in plasma and markedly increased lysine.

Concentrations of amino acids and urea in the plasma of the ruminating calf and estimation of the amino acid requirements

1. A study was made of factors affecting the plasma concentrations of free amino acids (PAA) and urea (PU) in calves receiving approximately equal daily amounts of concentrates (flaked maize and protein supplements) and straw, the former at 10.00 and 17.00 hours, the latter at 17.00 hours only.2. For calves receiving a diet containing 20 g nitrogen/kg dry matter in which the protein supplement was decorticated, extracted groundnut meal (DCGM) (diet A) there were marked increases in PAA and PU about 1–2 h after a morning feed, then a fall in these values 2 h later to a level which was maintained for the next 3 h. No similar changes occurred after the evening feed. Samples taken 3 h after the morning feed were used in subsequent comparative experiments. There was much more variation between animals than within animals in total PAA, PU and the concentrations of most individual amino acids in these samples.3. Total PAA and most individual amino acid concentrations were not appreciably affected when the DCGM intake was reduced to give 10 g N/kg dry matter in the diet (diet C), but PU was halved. When maize gluten replaced DCGM as the protein supplement at the higher N intake (diet B) then PU doubled, but again total PAA and most individual amino acid concentrations were little affected. Exceptions were arginine, which was halved, and leucine, which was doubled.4. Infusions of more than 4·4 g L-methionine/d into the abomasums of calves (110–160 kg live weight) receiving diet A led to a marked increase in plasma methionine concentration. This was considered to correspond with the point at which methionine requirements were met. Using a chromic oxide marker to estimate flows of methionine and cystine from the rumen to the duodenum, it was calculated that under these conditions the methionine requirement was 9·8 g/d, with a cystine flow of 4·9 g/d. Similar calculations showed the corresponding value to be 7·5 g/d with a cystine flow of 2·8 g/d for calves receiving diet C.5. Infusion of increasing levels of L-lysine into the abomasums of calves (110–160 kg live weight) receiving diet B led to a progressive increase in plasma lysine concentration. There was no consistent change in the rate of increase with increasing amounts infused. Estimated lysine requirement appeared therefore to be less than the flow of lysine from the rumen to the duodenum under these conditions (18·8 g/d).

Defective metabolic clearance of plasma arginine and ornithine in lysinuric protein intolerance

Fasting plasma lysine, arginine, and ornithine concentration is clearly statistically decreased in patients with lysinuric protein intolerance (LPI). In seven LPI patients and five reference subjects, one suffering from classic cystinuria, arginine and ornithine were infused intravenously at a stepwise-increased rate. Plasma steady-state concentration of the infused diamino acid was larger in LPI patients than in the reference subjects, although the patients lost more into the urine. The increase in plasma ornithine and urea concentration on arginine infusion was similar in both groups. Calculated metabolic clearance rates (MCR) of arginine and ornithine were clearly smaller in LPI patients than in the reference subjects. Infusion of lysine brought about a clear decrease in the MCR of arginine; this was similar in both groups of subjects. These findings suggest a defect in the transport of diamino acids into liver cells, such as has been earlier demonstrated in the renal tubuli. Such a general cellular transport defect of diamino acids would explain both basic characteristics of LPI: renal loss of diamino acids and slow urea synthesis.

Intravenous energy infusion in lambs: effects on nitrogen retentin, plasma free amino acids and plasma urea nitrogen

1. Acetate, propionate, butyrate and glucose were each administered intravenously (2.09 MJ (0.5 Mcal)/d) to lambs on either high-concentrate (HC) or high-roughage (HR) diets in two experiments with 4 × 4 Latin square design. During a control period all animals received a saline infusion.2. In lambs on the HC diet glucose was the most effective metabolite tested for improving nitrogen retention. Acetate was 51%, butyrate, 75%, propionate, 90% as effective as glucose in promoting N retention. Propionate infusion was most effective in increasing N retention in lambs on the HR diet. Acetate was 60%, butyrate, 75%, and glucose, 80% as effective as propionate. Glucose infusion did not lower plasma urea N (PUN) concentration in lambs receiving the HC diet, but the three volatile fatty acids (VFA) did lower it (P&lt; 0.01). All energy sources significantly reduced the PUN concentrations with the HR diet, but differences between sources were not significant.3. Plasma concentrations of valine, isoleucine, leucine and phenylalanine were depressed to 57, 71, 89 and 90% of their control values during glucose, propionate, butyrate and acetate infusion. Plasma concentrations of lysine and methionine were significantly decreased from control values (P&lt; 0.05) but there was no significant difference between energy sources. Lysine, valine, isoleucine and leucine were significantly (P&lt; 0.01) higher in lambs given the HR diet than in lambs given the HC diet. The concentration of alanine was greater (P&lt; 0.05) during glucose infusion than during acetate and butyrate infusions, indicating net synthesis of alanine and possibly other non-essential amino acids during glucose infusion, since the plasma free amino acid N concentration increased despite decreased concentrations of plasma essential amino acids during glucose infusion.4. The results indicated that glucose and propionate were superior to acetate and butyrate as energy sources for protein formation.5. The poor relationship between PUN and N balance and plasma free essential amino acids suggested that PUN concentration may have been inappropriate to assess the protein status under some dietary conditions.

Response of adult rats to deficiencies of different essential amino acids

1. Adult rats were fed on diets free of either lysine, methionine, threonine or protein. The threonine- and protein-deficient animals lost weight at approximately the same rate, about 100 g in 14 weeks, at which time several were moribund. In contrast, lysine-deficient animals lost only about 30 g in 14 weeks and had lost only 46 g after 22 weeks, when they were killed. Methionine-deficient animals showed an intermediate response. Losses in weight of several tissues – kidney, heart and two muscles – were related to, but not necessarily proportional to, the loss of body-weight. Liver weights relative to body-weights were large in lysine- and threonine-deficient animals and smallest in methionine-deficient animals.2. Adult rats were fed on diets containing zero, a moderate amount (about twice the estimated minimal requirement) or an excess (about four times the estimated requirement) of lysine or threonine in all combinations (3 × 3 design). Analysis of variance of the body-weights, tissue weights and tissue nitrogen contents indicated, in general, a significant effect of each amino acid, as expected, but also, in most instances, a significant interaction. Plasma concentrations of lysine and threonine were affected by the intakes of the respective amino acids, but plasma lysine concentrations were also affected by the threonine intake.3. Liver histology also suggested significant interactions between the two amino acids. Animals given no lysine but moderate amounts of threonine developed severely fatty livers; next most severely affected were animals receiving excess of both amino acids. Threonine deficiency, in the presence or absence of lysine, produced moderately fatty livers similar to those seen in protein-deficient animals.4. Since animals have varying ability to conserve body nitrogen when they are fed on diets limiting in different essential amino acids, measurements of biological value (BV) and net protein utilization by conventional methods, over a short period of time, over-estimate nutritive value relative to amino acid score and probably over-estimate the true nutritive value of poor-quality proteins, particularly those limiting in lysine. If so, this is a serious error, since it leads to underestimates of the protein requirements if BV is used. The fact that certain tissues, particularly the liver, do not necessarily lose nitrogen in proportion to total body nitrogen and may show specific pathological effects depending on the limiting amino acid or the proportions of amino acids in the diet also indicates that general measures of nitrogen economy may not be sufficiently discriminating tests of the nutritive value of proteins.

Lysine infusion in cystinuria: theoretical renal thresholds for lysine.

1. The renal handling of lysine was studied during the intravenous infusion of a lysine load given in such a way that the plasma lysine concentration was constantly increasing. Theoretical renal thresholds, and maximum rates of renal tubular reabsorption of lysine, have been determined in three normal adults, five homozygous cystinuric patients, and three persons, two ‘completely’ and the third ‘incompletely’ recessive, heterozygotes for cystinuria. All the cystinuric patients were found to have thresholds at less than 1 mg of lysine/100 ml of plasma; the thresholds of all three heterozygotes fell within the same range as those of the three normal persons, that is, at values only two to three times the fasting plasma lysine concentration. 2. Not only was the lysine load shown to greatly increase the excretion of cystine, arginine and ornithine in the normal and heterozygous persons, but it was also shown to increase the already high clearance of these three amino acids in three of the homozygous cystinuric patients. 3. It was shown that a water diuresis alone will not cause increased excretion of any amino acids except taurine.

Enzyme defect in saccharopinuria

Saccharopinuria, a newly found disorder of lysine metabolism in man, has so farbeen described in two slightly or moderately retarded patients. In each the fasting plasma lysine concentration is about ten times normal, and each one excretes large amounts of lysine and saccharopine (epsilon-N-[L-glutaryl-2]-L-lysine) in the urine. The metabolic defect, localized in the main degradative pathway of lysine, has been determined in the present experiments with cultured fibroblasts from our patient, a 4-year-old girl with spastic diplegia and mental retardation. The activity of the saccharopine-degrading enzyme, aminoadipic semialdehyde-glutamate reductase, was found to be about 40 per cent of that present in normal control cells. This defect was confirmed by assay of the activity of the same enzyme in muscle from the patient and from two control subjects.

The relationship between weight gain and free amino acid concentration of plasma and liver in rats fed a diet supplemented with various amounts of lysine.

This study was conducted to determine the effect of feeding graded levels of dietary lysine on weight gain and free amino acid concentra-tions in blood plasma and liver of weanling rats. Animals were fed on diets containing 11.6% wheat gluten (equivalent in nitrogen to a 10% casein diet) supplemented with graded levels of L-lysine HCl (0 to 10%) for 14 days. An outline of the results obtained follows: 1) Maximum weight gain was observed with the groups fed the lysine supplement in the 0.64 to 1.8% range. Growth declined with further in-crease of dietary lysine. 2) Blood and liver were sampled after decapitation 6 hr after the final feeding on the 14th day, and amino acid concentrations were determined. Plasma lysine concentrations rose rapidly as dietary lysine increased and reached the maximum level with the 1.8% lysine supplementation. With the large supplement diet, the lysine con-centrations maintained a plateau. Plasma threonine levels were high when dietary lysine was low. 3) Similar responses of lysine and threonine concentrations in liver to the increase of dietary lysine was also observed, but the effect was markedly less than those of plasma. Most other amino acids in plasma and liver were nearly unchanged even when the dietary lysine was varied over a wide range.

Plasma Amino Acid Response Curve and Amino Acid Requirements in Young Men: Valine and Lysine

A total of 14 young men were studied to determine the relationships between the concentration of free valine or lysine in blood plasma, and the dietary intake and daily requirement for each of these amino acids. The diet, containing an L-amino acid mixture, which was patterned as in egg protein, supplied nitrogen equivalent to about 0.5 g egg protein (N × 6.25)/kilogram body weight/day. In three separate experiments with a total of 10 subjects, dietary valine intake varied between 4 and 20 mg/kilogram body weight, and in another with four subjects, lysine intake ranged from 1 to 24 mg/kilogram body weight. The free valine level in fasting plasma was relatively constant at valine intakes below 14 to 16 mg/kilogram body weight/day and increased above this intake level. Crude nitrogen balances in these subjects approached equilibrium or were positive at valine intakes of about 12 to 14 mg/kg. Considering integumental and sweat nitrogen losses, it appears that fasting plasma valine levels begin to rise at an intake of 14 to 16 mg/kg, which is presumably the minimum requirement for these subjects. This technique should be useful to determine valine requirements under various conditions. In the lysine study three subjects remained in nitrogen balance throughout. Mean fasting plasma lysine levels were relatively constant at and below an intake of 6 mg/kilogram body weight and were 13 and 18% higher at the 7 and 24 mg/kg lysine intakes, respectively. In comparison with the valine findings, fasting plasma lysine levels would not appear as useful a response criterion to assess the human maintenance requirement for this amino acid.

Hyperlysinuria with hyperammonemia. A new metabolic disorder.

Dibasicaminoaciduria with hyperammonemia was observed in a mentally and physically retarded patient. Oral loading test results revealed diminished capacity to absorb lysine compared to five normal controls and served to distinguish this patient from those with familial protein intolerance. Fasting plasma arginine and lysine concentrations were decreased to 10% to 50% of normal values, presumably because of decreased intestinal absorption and increased renal excretion. The severe postprandial hyperammonemia may be due to a lack of substrate (arginine) for urea cycle activity. Arginine and lysine deficiency may impair protein synthesis and this factor may be involved in the observed physical and mental retardation. The patient is an example of a previously unrecognized syndrome characterized by dibasicaminoaciduria with defective intestinal absorption of lysine, severe postprandial hyperammonemia with seizures, mental retardation, and subnormal growth and physical development.

Influence of site of sampling and stage of fast on concentrations of all free amino acids in the plasma and liver in the young pig.

TYPPO et al. (1970) and Nordstrom et al. (1970) reported that peak levels of free essential amino acids in the plasma of young swine were reached at 2 to 4 hr. post-feeding. Free essential amino acids in the plasma had attained baseline levels by 8 to 12 hr. postfeeding, except for amino acids that were present in the diet in excess. Some amino acids increased in the plasma during the period from 18 to 36 hr. of fast, apparently as a result of tissue protein catabolism. Patterns for concentrations of isoleucine, leucine, lysine, methionine, threonine and valine in plasma from the portal vein, abdominal aorta, posterior vena cava and jugular vein were similar during the posffeeding period (Nordstrom et al., 1970). With allowance for dilution of blood as it enters the general circulation, removal, of the six amino acids by the liver was indicated by the greater levels of amino acids in the portal vein than the abdominal aorta during the first 6 to 8 hr. post-feeding.

The effect of urea in the diet of the early-weaned calf on weight gain, nitrogen and sulphur balance, and plasma urea and free amino acid concentrations

Experiments were conducted with 40 male Friesian calves between 5 and 11 weeks of age. The control diet contained meat meal as the sole protein supplement to grain, and this was replaced by urea to supply 20.1, 39.2, or 55.6% of the dietary nitrogen. The growth rate of the calves, when corrected for digestible dry matter intake, was significantly lower when urea supplied 55.6 % of the dietary nitrogen than when it supplied 0, 20.1, or 39.2% of the dietary nitrogen. The retention of nitrogen and sulphur was lower in the calves given 55.6% of the dietary nitrogen as urea, but the ratio of nitrogen to sulphur retained was constant for all diets. There was a positive correlation between dietary urea nitrogen intake and urea nitrogen concentration in the blood plasma. The urea nitrogen and a-amino nitrogen concentration in the blood plasma was greater in calves at 2 weeks of age than at 11 weeks of age. The concentrations of free glycine, valine, leucine, ornithine, lysine, and histidine in the blood plasma were significantly lower in the calves given the higher levels of urea in the diet.

Comparison of fasting plasma amino acid patterns in the pig, rat, and chicken.

Comparison of free amino acid levels in plasma (PAA) at 8, 12, 16, 20, and 24 h postprandial in pigs, rats, and chickens revealed differences between mammalian and avian species in the fasting metabolism of amino acids. PAA patterns during fasting were more variable for chickens than for either rats or pigs. There was a marked increase in the plasma lysine level in chickens (27.5 to 115.8 μg/ml) as fasting was extended from 8 to 24 h. Plasma lysine levels in pigs and rats, by contrast, increased from 14.2 to 19.5 μg/ml and 39.8 to 63.7 μg/ml respectively during the same period of fast. Plasma threonine and methionine levels also increased in chickens during fasting whereas the levels of these amino acids in pigs and rats decreased or remained unchanged during a 24-h fast. Extending the period of fast to 36 h in chickens and 48 h in pigs resulted in further small increases in the plasma lysine concentration.

Effect of Iodinated Casein and Thiouracil on Free Amino Acids in the Blood Plasma of Chicks

IN A previous paper (Shao and Hill, 1968) it was reported that thiouracil as a dietary supplement reduced the concentration of free lysine, threonine, tryptophan, methionine and leucine in the blood plasma of chicks. The present paper gives the results of experiments in which thiouracil and/or iodinated casein were added to a basal diet and extends the observations to several additional amino acids.EXPERIMENTALThe basal diet, essentially a wheat, corn soybean meal mixture, was described in detail by Shao and Hill (1968). White Plymouth Rock pullets were fed this diet from day old to 7 days of age and then divided on the basis of weight into lots of 12 chicks each. The experimental diets were assigned to these lots at random, each diet to 4 lots and the experiments continued for another 14 to 15 days. Blood samples were removed from each chick in two lots per treatment…