Hepatic Histidase Research Articles

Dietary Nitrogen Intake Regulates Hepatic Malic Enzyme Messenger Ribonucleic Acid Expression

Increased dietary protein intake rapidly (3 h) decreases malic enzyme and increases hepatic histidase mRNA expression. Experiments were conducted to determine the role that individual dispensable amino acids and nonprotein N sources might have in regulating the activity of these enzymes and to determine if the addition of a N supplement to a practical broiler diet during the entire rearing period would reduce abdominal fat accumulation in broilers. Broiler chicks were fed a basal diet containing 22% protein or this diet supplemented with 9.5% l-Glu, 5% Gly, 6% l-Ala, 5.08% ammonium bicarbonate, or 4.25% dibasic ammonium phosphate for 24 h. Each of the dietary supplements added 0.90% total N to the diet. Hepatic malic enzyme mRNA expression was significantly (P < 0.05) depressed in chicks fed any of the supplemented diets compared with chicks fed the basal diet. Histidase mRNA expression, however, was only significantly increased in the chicks fed the basal diet supplemented with Gly. Broilers fed practical corn-soybean meal starter and developer diets supplemented with 2.3, 4.7, or 9.5% Glu from 0 to 40 d of age had significantly smaller abdominal fat pads relative to BW than broilers fed the unsupplemented corn-soybean meal diets. Feeding the Glu supplements, however, reduced the overall BW gain of broilers by 100 to 150 g compared with broilers fed the unsupplemented diets. The results suggest that hepatic mRNA expression of malic enzyme may be regulated by total dietary N intake, whereas hepatic mRNA expression of histidase may be regulated by specific amino acids.

The Role of Glucagon in Regulating Chicken Hepatic Malic Enzyme and Histidase Messenger Ribonucleic Acid Expression In Response to an Increase in Dietary Protein Intake

Increased dietary protein intake rapidly (3 h) decreases hepatic malic enzyme and increases hepatic histidase mRNA expression in broiler chicks. A series of experiments was conducted to determine the role that glucagon or a specific mixture of dietary amino acids might have in regulating the rapid changes in mRNA expression of these enzymes, when dietary protein intake is increased. Three hours after the injection of glucagon (240 μ/kg of BW) into the brachial vein of broiler chicks, hepatic malic enzyme mRNA expression was significantly lower and hepatic histidase mRNA expression was significantly greater than the level detected in saline-injected chicks. In addition, broiler chicks fed a high (40 g/ 100 g of diet) protein diet had significantly higher plasma glucagon levels at 1 and 3 h after initial access to this diet than broiler chicks fed a basal (22 g/100 g of diet) protein diet. The plasma glucagon concentration, however, was not different between the chicks fed the 2 dietary protein levels at 2 h after the initial access to the 2 diets. When a mixture of indispensable or dispensable amino acids was added to the basal diet to equal the concentrations of the individual indispensable or dispensable amino acids in the high protein diet, hepatic mRNA expression of malic enzyme and histidase were intermediate to the expression found in chicks fed the basal and high protein diet. The results indicate that glucagon may mediate the changes in the mRNA expression of malic enzyme and histidase in response to dietary protein intake and that total amino acid intake rather than the ingestion of specific amino acids regulates the mRNA expression of malic enzyme and histidase in chicks.

Molecular cloning of chicken hepatic histidase and the regulation of histidase mRNA expression by dietary protein

Chicken hepatic histidase activity varies with dietary protein consumption, but the mechanisms responsible for this alteration in activity are unclear. In the present research, the complete coding sequence and deduced amino acid sequence for chicken histidase was determined from clones isolated from a chicken liver cDNA library. The deduced amino acid sequence of chicken histidase has greater than 85% identity with the amino acid sequences of rat, mouse, and human histidase. In a series of four experiments, broiler chicks were allowed free access for 1.5, 3, 6, or 24 h to a low (13 g/100 g diet), basal (22 g/100 g diet) and high (40 g/100 g diet) protein diet. In the final experiment 5, chicks were allowed free access for 24 h to the basal, high protein diet or the basal diet supplemented with three different levels of l-histidine (0.22 g/100 g diet, 0.43 g/100 g diet or 0.86 g/100 g diet). There were no differences in the expression of the mRNA for histidase at 1.5 h, but at 3 h, histidase mRNA expression was significantly ( P<.05) greater in chicks fed the high protein diet compared to chicks fed the low protein diet. At 6 and 24 h, histidase mRNA expression was significantly enhanced in chicks fed the high protein diet, and significantly reduced in chicks fed the low protein diet, compared with chicks fed the basal diet. Histidase mRNA expression was not altered by supplementing the basal diet with histidine. The results suggest that previously observed alterations in the activity of histidase, which were correlated to dietary protein intake, are mediated by rapid changes in the mRNA expression of this enzyme, and are not necessarily related to dietary histidine intake.

Hepatic histidase and muscle branched chain aminotransferase gene expression in experimental nephrosis

Protein and amino acid metabolism is altered during nephrotic syndrome. However, the expression of the amino acid degrading enzymes has not been well studied. The objective of this work was to assess the expression of hepatic histidase (Hal) and skeletal muscle mitochondrial branched chain amino transferase (BCATm) in rats with experimental nephrotic syndrome induced by a single injection of puromycin aminonucleoside (150 mg/kg). Six days after the injection rats were killed and hepatic Hal and skeletal muscle BCATm activities were measured. Also, total mRNA from both tissues was isolated and Hal and BCATm mRNA expression were analyzed by Northern blot. Rats with NS showed a reduction in food intake with respect to the control group. Hepatic Hal activity increased significantly in nephrotic and pair fed rats by 59% compared to control group. This change in activity was associated with a corresponding increase in Hal mRNA abundance. On the other hand, skeletal muscle BCATm activity and mRNA abundance were similar in the three groups studied. These results suggest that the increase in Hal expression was associated with the reduced food intake and not to the NS. However, BCAT expression did not change indicating the importance of BCAA in body nitrogen conservation.

Histidine-Imbalanced Diets Stimulate Hepatic Histidase Gene Expression in Rats

A high protein concentration in the diet induces the gene expression of several amino acid degrading enzymes such as histidase (Hal) in rats. It is important to understand whether the amino acid pattern of the dietary protein affects the gene expression of these enzymes. The purpose of the present work was to study the effect of a histidine-imbalanced diet on the activity and mRNA concentration of rat hepatic histidase. Seven groups of six rats were fed one of the following diets: 1) 6% casein (basal), 2) 20% casein, 3) 35% casein, 4) an imbalance diet containing 6% casein plus a mixture of indispensable amino acids (IAA) equivalent to a 20% casein diet without histidine (I-20), 5) 6% casein plus a mixture of IAA equivalent to a 35% casein diet without histidine (I-35), 6) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 20% casein diet, 7) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 35% casein diet. Serum histidine concentration was inversely proportional to the protein content of the diet, and it was significantly higher in rats fed the corrected diets compared to their respective imbalanced diet groups. Hal activity increased as the protein content of the diet increased. Greater histidine imbalance resulted in lower food intake and higher Hal activity. Rats fed histidine-corrected diets had lower activity than their respective imbalanced groups. Differences in Hal activity were associated with differences in the concentration of Hal mRNA. These results indicate that rats fed a histidine-imbalanced diet exhibit reduced food intake and weight gain and increased Hal gene expression as a consequence of an increased amino acid catabolism.

Hepatic Histidase Gene Expression Responds to Protein Rehabilitation in Undernourished Growing Rats

We studied the effect of nutritional rehabilitation with a 6, 18 or 50% casein diet in undernourished rats on histidase (Hal) expression. Undernutrition was induced by feeding rats a 0.5% casein diet for 5 wk. Over this period, growth, serum total proteins and insulin-like growth factor-I (IGF-I) levels were significantly lower than those of rats that freely consumed an 18% casein diet. During this period, undernutrition also significantly reduced Hal activity and Hal-mRNA concentration. Nutritional rehabilitation for 21 d with a 6% casein diet did not change any of these variables. Nutritional rehabilitation with an 18 or 50% casein diet for 1 d initiated the restoration of Hal activity and mRNA concentration. After 10 d of consuming 18 or 50% casein diets, Hal activity was 5- and 14-fold, and mRNA concentration was 8.5- and 23-fold higher, respectively, than in the protein-undernourished group (PU). During this period, body weight, total serum proteins and IGF-I levels were also significantly (P < 0.05) higher than those of the PU group. At the end of 21 d of rehabilitation with an 18 or 50% casein diet, Hal activity was 14- and 31-fold higher and Hal mRNA concentration was 10- and 24-fold higher, respectively, than in the PU group. In conclusion, our data showed that rehabilitation of undernourished rats with a 6% casein diet was not sufficient to re-establish growth indicators, Hal activity or gene expression, and that nutritional rehabilitation with an 18 or 50% casein diet effectively re-established body weight , biochemical variables and the capacity of histidase gene expression to eliminate the excess of protein.

Regulation of histidase gene expression by glucagon, hydrocortisone and protein-free/high carbohydrate diet in the rat

The effect of glucagon and hydrocortisone was investigated to understand the mechanism of induction of hepatic histidase gene. In this study, glucagon (0.6 mg 100 g body wt d ) was injected to rats fed 10% casein diet. After 3h of the last injection, histidase activity and amount of enzyme were induced by 3 fold and histidase mRNA concentration by 6 fold. Injection of hydrocortisone (2 mg 100g body wt d ) increased 100% histidase activity and mRNA concentration and by 150% the amount of enzyme after 3 h of the last injection. These results indicate that glucagon is a better inductor of histidase gene expression than hydrocortisone. Another purpose of the study was to evaluate if a protein-free/high carbohydrate diet could reverse the induction of Hal expression produced by a high protein diet. Hal activity, amount of enzyme and mRNA concentration was repressed by 68, 88 and 95% respectively by a protein-free/high carbohydrate diet. Injection of glucagon reversed partially the effect of a high carbohydrate diet, however, injection of hydrocortisone under the concentration used in these experiments did not reverse the effect of a high carbohydrate diet. These results support the evidence that hepatic histidase gene expression is probably regulated transcriptionally by hormones.

Histidase Expression Is Regulated by Dietary Protein at the Pretranslational Level in Rat Liver

The effect of dietary protein on the expression of histidase (Hal) was investigated to understand the mechanism of induction of histidase by a high protein diet. In this study, we examined the following: 1) the effect of 0, 6, 18, 35 and 50% casein diets on hepatic and epidermal Hal activity, amount of the enzyme and Hal-mRNA; 2) the effect of a high histidine diet (1.25%) on Hal expression; 3) the response of Hal expression in rats fed a 10% casein diet and injected with glucagon (0.6 mg /(100 g body wt⋅d); and 4) the half-lives of the enzyme and Hal-mRNA in rats fed an 80% casein diet for 7 d followed by a protein-free diet. Hal activity increased as the protein content in the diet increased (r = 0.986, P < 0.001) and was associated with a significant increase in Vmax without a change in Km. The dietary regulation was liver specific because skin Hal was unresponsive. Increments in hepatic Hal activity were accompanied by concomitant significant increases in the amount of histidase and its mRNA. The response was more pronounced in rats fed diets containing >18% casein. Rats fed a 12% casein diet containing 1.25% histidine did not have different Hal activity and mRNA levels compared with rats fed a 12% casein diet, indicating that Hal expression is not modified by its substrate. Injection of glucagon into rats fed the 10% casein diet increased Hal activity threefold and Hal- mRNA expression fivefold compared with uninjected rats fed the same diet. The apparent half-life of hepatic histidase in protein-depleted rats previously fed an 80% casein diet was 2.8 d, whereas the half-life of Hal-mRNA was 17 h. In summary, these data support the hypothesis that Hal expression is regulated by dietary protein at the pretranslational level in rat liver, and that glucagon is one of the hormones involved in the induction of Hal.

Effect of Hormones and Protein Level on Induction of Enzymes Involved in Histidine Degradation and Folate Metabolism

Summary The effects of the hormones thyroxine, glucagon and hydrocortisone, and of protein level on hepatic histidase, urocanase, and certain folate enzymes were studied in rats. Hypothyroidism, induced either by feeding thiouracil or by thyroidectomy, increased histidase, urocanase, and the folate dependent enzyme formiminotransferase, and increased histidine metabolism as measured by oxidation of the 2-ring carbon of histidine to carbon dioxide. Hyperthyroidism, produced by feeding thyroid powder, decreased histidine oxidation and liver levels of this enzyme. Glucagon and hydrocortisone increased histidine oxidation and increased levels of histidase, urocanase and formiminotransferase. Elevated levels of casein and soy protein produced four to eight fold increases in histidase and urocanase, and 50% increases in formiminotransferase. Thus, increases in histidase and urocanase involved in the formation of formiminoglutamic acid (FIGlu) were accompanied by increases in the FIGlu metabolizing enzyme formiminotransferase.

Sex-differentiated enzyme levels and oestrogen uptake in adult rats treated neonatally with tamoxifen or CI-628

Serum cholinesterase, hepatic histidase and monoamine oxidase activity levels are higher in adult female rats than in adult male rats. Exposure of neonatal rats to antioestrogen (tamoxifen or CI-628) resulted in increased serum cholinesterase in adult females only and no effect on hepatic histidase and monoamine oxidase in both sexes. Neonatal tamoxifen or CI-628 treatment resulted in reduced body weights in adult male rats and reduced uterine wet weights in adult female rats. Circulating oestrogen levels measured in adult female rats treated neonatally with tamoxifen were not significantly different from controls. Specific oestrogen uptake in the brain of adult male and female rats was found to be higher in the pituitary than in the preoptic-anterior hypothalamic area and the median eminence-basal hypothalamus than in the cerebral cortex. There was higher uptake of [ 3H]oestradiol-17β in male pituitaries than in female pituitaries. No other sex-difference was observed. Neonatal tamoxifen treatment did not alter the capacity of these brain tissues to take up oestrogen. It is suggested that neonatal antioestrogen exposure has altered the endocrine expression of serum cholinesterase in adult female rats by interfering with normal imprinting mechanisms.

Endocrine regulation of sex differences in hepatic histidase activity.

Hepatic histidase activity in adult female rats is twice that in adult male rats. Hypophysectomy and thyroidectomy result in a significant increase in hepatic histidase activities in males, but not in females. This effect on histidase is reversed by the exogenous administration of tri-iodothyronine, but not by ectopic pituitary glands or purified pituitary hormones.

Effect of dietary electrolytes and histidine on histidine metabolism and acid-base balance in rainbow trout ( Salmo gairdneri)

1. 1. Rainbow trout fingerlings were fed diets containing 1.2, 1.8 and 2.6% histidine and two mixtures of Na, K and Cl ( Na + K - C1 = 0 or −200 meq/ kg diet) in a factorial design. 2. 2. Growth and efficiency of feed conversion were not affected by histidine in the diet when it contained the −200 meq/kg electrolyte mixture, but with the 0 meq/kg level, 2.6% histidine depressed both measures of response. 3. 3. Histidine increased plasma and muscle histidine levels, increased hepatic histidase activity, but did not affect hepatic histidine-pyruvate aminotransferase activity. 4. 4. Muscle-free histidine concentrations were markedly higher and lysine concentrations were lower in trout receiving 0 meq/kg than those receiving the −200 meq/kg electrolyte mixture. 5. 5. The electrolyte balance of the diet has a marked effect on the metabolism of histidine in trout.

Histidase mRNA. Nature of translational products, tissue specificity, and differential development in male and female rat liver.

Histidase is expressed in only two tissues of the rat, liver and epidermis. Hepatic histidase synthesis and catalytic activity undergo complex sex-specific developmental courses. To determine whether changes in functional histidase mRNA levels underlie this developmental pattern, total cellular RNA was translated in a rabbit reticulocyte cell-free lysate system. Adult liver total cellular RNA directed the synthesis of three translational products immunoreactive with anti-native histidase: a polypeptide of Mr = 75,000 (75K), which corresponds to the in vivo synthesized histidase subunit, and two higher molecular weight proteins, a major and a minor peptide of Mr = 150,000 (150K) and 140,000 (140K), respectively. These latter peptides do not seem to be aggregates or dimers of the 75K polypeptide or precursors which are post-translationally hydrolyzed to Mr = 75,000; their origin and function remain to be clarified. In contrast to in vitro translation of hepatic total cellular RNA, Western blot analysis of liver cytosol confirms the presence of only the 75K histidase subunit, with no evidence of anti-histidase immunoreactive peptides of Mr = 140,000-150,000 synthesized in vivo. Quantitation of the radioactivity in the immunocomplexed 75K histidase subunit, translated using total RNA from livers of fetuses, 19-day-old males, 35-day-old males, adult males and females, and adult kidney and brain (0, 0.007, 0.010, 0.016, 0.031, 0, and 0%, respectively, of total released proteins) indicates that, in general, levels of functional histidase subunit mRNA reflected histidase catalytic activity (0, 0.20, 0.50, 1.01, 3.00, 0 and 0 units/g of tissue) during tissue differentiation and sex specific development. The above data indicate that initial expression and subsequent increases in synthesis and activity of histidase during hepatic differentiation, postnatal development, and sex hormone regulation are due to pretranslationally controlled augmentation in the levels of functional mRNA which specifies the histidase subunit. In tissues which do not express histidase no functional histidase mRNA is evident. The levels of the RNA which translate the combined 140-150K histidase-like polypeptides (0, 0.007, 0.014, 0.035, 0.034, 0, and 0% of released proteins) also paralleled the increase in enzymatic activity during tissue differentiation and development; however, no difference between males and females was evident. The significance of these observations awaits elucidation of the nature of these RNA(s).

Effects of hypophysectomy and triiodothyronine on de novo biosynthesis, catalytic activity, and estrogen induction of rat liver histidase.

Synthesis of rat liver histidase is under multihormonal control; estrogen, glucocorticoid, and glucagon, via cAMP, induce this enzyme. By means of in vivo [3H]leucine incorporation into immunoprecipitated histidase, relative to that incorporated into total soluble protein, we have now demonstrated that de novo hepatic histidase synthesis, as well as catalytic activity, is selectively increased following hypophysectomy. Treatment of hypophysectomized rats with physiological levels of triiodothyronine (T3) diminished histidase synthetic rates and catalytic activities to normal levels, despite concomitant elevation in total soluble protein synthesis. Thyrotoxic doses of T3 further reduced histidase synthesis to barely measurable rates. Since thyroid hormone is under pituitary regulation, this hormone may be primary in the hypophyseal suppression of histidase. Estrogen does not induce hepatic histidase in the hypophysectomized rat. However, administration of the histidase suppressor, T3, or prolactin, which in itself has no effect on this enzyme, was ineffective in reinstating estrogen inducibility of histidase in the hypophysectomized animal. Thus, some as yet unknown hypophyseal agent is required for estrogenic inducibility of this liver enzyme.

Metabolism of histidine in growing rats at various dietary protein levels.

The metabolic fate of the carbon skeleton of l-(U-14C)-histidine has been investigated in growing rats fed diets containing different percentages of protein calories (0, 5, 10, 15 and 30 PC%) at 410 kcal of metabolizable energy per 100 g diet.The incorporation of 14C into body protein at 12 hr after injection of 14C-histidine was about 70% of the dose in rats fed 0 to 10 PC% diets, though the value was reduced in rats fed higher PC% diets. The expired 14CO2 production was depressed in the low protein groups, and it showed an inverse pattern to that of 14C incorporation into body protein. Urinary excretion of 14C was about 20% of the dose in all dietary groups. The activities of hepatic histidase, urocanase and histidine-pyruvate aminotransferase were increased in the 30 PC% group.These results indicate that the metabolic response of histidine to dietary protein changes around 10 PC%, where growth rate and body protein retention reached approximate plateaus.The nutritional significance of the metabolism of...

Neonatal estrogen treatment alters sexual differentiation of hepatic histidase.

Hepatic histidase was used as an enzyme marker for the study of neonatal programming in intact rats. Diethylstilbestrol (DES) or 17 beta-estradiol (E2) treatment for days, 2, 4, and 6 post partum resulted in decreased histidase activities in the adult female, but no effect was seen in prepubertal male and female rats or in adult males. In contrast, similar neonatal doses of testosterone propionate (TP) had no effect on histidase. Dose-response experiments demonstrate a 3-fold greater neonatal sensitivity to DES than to E2. The action of neonatal estrogen treatment is demonstrated to be permanent and irreversible. Neonatal treatment with E2, DES, or TP resulted in decreased uterine wet weights in adult females (E2 less than DES less than TP less than controls). Circulating sera estrogen levels were lower in adult E2- and DES-treated females than in TP-treated and control females. Our results suggest that these alterations may be due to direct toxic effects on the postnatal development of the female reproductive tract and endocrine system and/or to organizational effects on nerve endings in the hypothalamus that result in programming for altered sexual differentiation of hepatic metabolism.

Programming of hepatic histidase following prenatal administration of diethylstilbestrol

Diethylstilbestrol was demonstrated to be a positive modulator of rat liver histidase in a manner similar to oestradiol-17β. Oral administration of diethylstilbestrol to pregnant rats at the 15th day of gestation and subsequent measurement of the sex-dependent activities of histidase in these offspring revealed no effect on histidase activities in immature male and female and adult male offspring. However, histidase activities of the adult female offspring were decreased and activities approached those of normal adult males. This masculinization of histidase activity can be reversed by exogenously administered estrogen.

Effect of Histidine Intake and Hepatic Histidase Activity on the Metabolism of Histidine in Vivo

The metabolic fate of histidine was investigated in young male rats fed diets containing from 0 to 0.6% of L-histidine and 14.7% of a mixture of other L-amino acids. Liver histidase activity was not influenced by dietary histidine content. Tissue histidine concentration increased gradually, then more sharply, as dietary histidine content was increased from 0 to 0.325% then it plateaued. During ad libitum consumption of these diets containing L-[U-14C]histidine, 14CO2 production remained low until dietary histidine content exceeded 0.25%, then it increased linearly until with 0.6% of dietary histidine 25% of the absorbed dose was oxidized. The proportion of absorbed 14C incorporated into tissue proteins was high when dietary histidine content was low and decreased as dietary histidine content increased. Histidine requirement estimated from the inflection point of the oxidation curve was in good agreement with values determined from growth response and urinary nitrogen excretion. Liver histidase activity increased almost linearly with increasing dietary casein content. Rats fed 12% casein oxidized more of a load of L-[ring-2-14C]histidine (500 mg/100 g rat) to CO2 than those fed 8% casein. Plasma histidine clearance rate after intraperitoneal injection of 50 mg histidine/100 g rat increased as the casein content of the diet was increased from 4% to 8%, 12% and 40%.

Effects of estrogen, glucocorticoid, glucagon, and adenosine 3':5'-monophosphate on catalytic activity, amount, and rate of de novo synthesis of hepatic histidase.

The mechanisms by which estrogen, glucocorticoid, glucagon, and adenosine 3':5'-monophosphate (cAMP), regulators which participate in the postnatal development of rat liver histidase, elevate the catalytic activity of this enzyme have been explored. A monospecific antibody against homogeneously purified preparations of rat liver histidase has been elaborated in the goat. Employing this antibody in immunotitration experiments, it has been demonstrated that the elevations of hepatic histidase activity elicited by administration in vivo of estradiol-17beta, cortisol acetate, glucagon, and N6,O2'-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cAMP) are paralleled, in each instance, by equivalent increments in immunoprecipitable histidase protein. Following administration of each of the three hormones and dibutyryl cAMP, rates of [14C]leucine incorporation in vivo into rat liver histidase, isolated by immunoprecipitation, relative to incorporation rates into total soluble hepatic protein, increase in magnitudes which are comparable to increases in enzyme amount and catalytic activity. It is thus inferred that estrogen, glucocorticoids, and glucagon, via cAMP, each regulate rat liver histidase development at specific postnatal stages by inducing increases in histidase biosynthetic rates.

Studies on the postnatal development of responses of hepatic histidase to endocrine control

Each of three known hormonal inducers of hepatic histidase ( l-histidine ammonia-lyase, EC 4.3.1.3), glucocorticoid, glucagon and estrogen, elicits a unique regulatory pattern during postnatal development of the rat: (1) In the male, glucocorticoid augments hepatic histidase activity during immaturity, the stage during which basal enzyme levels are low and rising; as maturity is approached and the enzyme has plateaued at its adult value, hepatic histidase becomes refractory to glucocorticoidal control. Hepatic histidase induction by adrenal corticoid also occurs in the immature female, however, thereafter there is a gradual inversion, in which glucocorticoid, by counteracting estrogenic induction of hepatic histidase, suppresses the high enzyme levels of the adult female. (2) In contrast, glucagon is capable of enhancing hepatic histidase at all postnatal stages in the male; in the intact adult female, however, glucagon also slightly suppresses this enzyme. Dibutyryl adenosine 3′,5′-monophosphate (dibutyryl cyclic AMP), administered with the inhibitor of its diesterase, theophylline, reproduces the inductive effects of glucagon. (3) Estrogen manifests yet a third developmental pattern of liver histidase regulation. During the first neonatal month, hepatic histidase is refractory to estrogenic induction, following which the liver develops competence to respond to estrogen with induced levels of the enzyme, an attribute which is maintained throughout postnatal life. Unlike estrogenic induction of hepatic histidase, which requires pituitary integrity, enhancement of this enzyme in immature animals by glucocorticoid and glucagon occurs in hypophysectomized animals. Administration of actinomycin D, cycloheximide or ethionine, each prevents induction of hepatic histidase by each of these hormones, indicating mandatory requirements for continued RNA and protein syntheses in each case. The three inducing hormones, injected at maximal dosages, are not additive, implying that certain common rate-limiting step(s), possibly transcriptional and/or translational, underly all three inductions.