Folic acid metabolism in vitamin B12-deficient sheep. Depletion of liver folates

1. Metabolism of folate was studied in six ewes in an advanced state of vitamin B(12) deficiency as judged by voluntary food intake and in their pair-fed controls receiving vitamin B(12). A group of four animals that were maintained throughout the experiment at pasture was also studied. 2. After 34-40 weeks on the cobalt-deficient diet urinary excretion of formiminoglutamate by four deficient animals was about 3.2mmol/day and this was not significantly decreased by injection of three of them with about 4.5mug of [2-(14)C]folate/kg body weight per day for 5 days. Three days after the last injection retention of [2-(14)C]folate by the livers of the deficient animals (5.5% of the dose) was lower than that of their pair-fed controls (26% of the dose) but there was no evidence of net retention of injected folate in the livers of either group. Urinary excretion of (14)C indicated that renal clearance of folate may have been impaired in very severe vitamin B(12) deficiency. 3. As estimated by microbiological assays total folates in the livers of animals at pasture (12.9mug/g) included about 24% of 5-methyltetrahydrofolate as compared with about 72% of a total of 12.5mug/g in three further ewes fed on a stock diet of wheaten hay-chaff and lucerne-chaff. Liver folates of vitamin B(12)-deficient animals (0.5mug/g) included about 88% of 5-methyltetrahydrofolate as compared with about 51% of a total of 5.2mug/g in pair-fed animals treated with vitamin B(12). 4. Chromatography of liver folates of the pair-fed animals permitted quantitative estimates of the pteroylglutamates present. The results showed that the vitamin B(12)-deficient livers were more severely depleted of tetrahydrofolates and formyltetrahydrofolates than of methyltetrahydrofolates and that as the deficiency developed they were more severely depleted of the higher polyglutamates than of the monoglutamate within each of these classes. Results from animals injected with [2-(14)C]folate indicated an impairment of the exchange between pteroylmonoglutamates and pteroylpolyglutamates in the livers of deficient animals. 5. In vitamin B(12)-deficient animals with food intakes below 200g/day some of the liver folates were not completely reduced and some degradation of pteroylpolyglutamates was detected. The latter condition may have been associated with fatty liver. 6. The results are discussed in relation to current theories of vitamin B(12)-folate interactions.     

Folic acid metabolism in vitamin B12-deficient sheep. Effects of injected methionine on liver constituents associated with folate metabolism

1. A study was made of the effects of injected l-methionine on the activity of several enzymes of folate metabolism, and on the transport of methotrexate in liver preparations from vitamin B(12)-deficient ewes and their pair-fed controls receiving vitamin B(12). 2. The activities of dihydrofolate reductase (EC 1.5.1.3) and 5-methyltetrahydrofolate-homocysteine transmethylase were significantly decreased in the liver of vitamin B(12)-deficient animals, but were unaffected by l-methionine. 3. The concentration of S-adenosyl-l-methionine in the liver of deficient animals was about one-half of that in normal animals, and was restored to normal by either vitamin B(12) or l-methionine. 4. Methylenetetrahydrofolate reductase (EC 1.1.1.68) from sheep liver was inhibited by S-adenosyl-l-methionine in vitro, but not by concentrations of S-adenosyl-l-methionine found in the liver of vitamin B(12)-deficient animals after injection of physiological amounts of l-methionine. 5. Pteroylpolyglutamate synthetase activity was significantly increased in the liver of vitamin B(12)-deficient animals, and was decreased by intravenous injections of l-methionine. 6. l-Methionine injections increased the initial rate of uptake of methotrexate in liver slices from deficient animals and acted synergistically with vitamin B(12) to increase the quantity taken up in 40min. The failure of folate metabolism in vitamin B(12) deficiency can be satisfactorily explained if l-methionine similarly affects the membrane transport of naturally occurring folates. 7. Further details of the results have been deposited as Supplementary Publication SUP 50028 (4 pages) at the British Library (Lending Division), (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies may be obtained on the terms given in Biochem. J. (1973) 131, 5.     

Methylmalonic acid and coenzyme A concentrations in the livers of pair-fed vitamin B12-deficient and vitamin B12-treated sheep

The concentrations of CoA in the livers of severely vitamin B(12)-deficient ewes were about 2.6 times those in pair-fed animals treated with vitamin B(12). When the feeding rates of the pair-fed animals were closely similar, the concentrations of methylmalonic acid in deficient livers were about twice those in vitamin B(12)-sufficient livers. The molar concentrations of CoA present were more than three times those of methylmalonic acid in both deficient and treated animals, and it is concluded that the elevated concentrations of CoA in the deficient livers were not primarily due to accumulation of methylmalonyl-CoA.     

Effect of high levels of dietary folic acid on folate metabolism in vitamin B12 deficiency

The effect of administering high levels of folic acid to vitamin B12-deficient animals was studied. In B12 deficiency histidine oxidation is decreased. This is the result of both decreased liver folate levels and increases in the proportion of methyltetrahydrofolates. The purpose of this study was to determine if the addition of very high levels of folic acid to B12-deficient diets could increase liver folates and thereby restore histidine oxidation. Rats were fed a soy protein B12-deficient diet containing 10% pectin which has been shown previously to accelerate B12 depletion. When this diet was supplemented with B12 and folic acid, histidine oxidation was 5.4% in 2 h and the livers contained 3.49 micrograms of folate/g. In the absence of B12, the histidine oxidation rate was 0.34% and the liver folate level was 1.33 micrograms/g. When 200 mg/kg of folic acid was added to the B12-deficient diet there was no increase in histidine oxidation (0.35%) but the liver folates were increased to 3.68 micrograms which is about the same as that with B12 supplementation. The percentage tetrahydrofolate of the total liver folates was the same with and without a high level of dietary folic acid. Thus there was an increase in the absolute level of tetrahydrofolate without any increase in folate function as measured by histidine oxidation. Red cell folate levels were the same with and without B12, which is in contrast to the markedly lower liver folate levels in B12 deficiency. These data suggest a difference between B12 regulation of folate metabolism in the liver and in the bone marrow.     

Vitamin B6 metabolism in Morris hepatomas

The enzymes involved in the metabolism of vitamin B6 were measured in Morris hepatomas and livers of female Buffalo rats fed pyridoxine-sufficient and deficient diets. Pyridoxal phosphate levels in plasmas hepatomas, and livers were also determined. Nontumor-bearing animals were maintained as controls. Regardless of the B6 nutritional status, the concentration of pyridoxal phosphate was lower in the hepatomas than in the livers of the host animals. The apoenzyme levels of ornithine decarboxylase, a pyridoxal phosphate-dependent enzyme, were higher in the hepatomas from animals fed the B6-deficient diet. Liver pyridoxine kinase activity was higher in B6-sufficient animals. In contrast, tumor pyridoxine kinase activity was influenced by B6 intake and was significantly lower than that in host liver. Liver pyridoxine phosphate oxidase activity was not significantly affected by B6 intake or by the presence of tumor. In contrast, hepatomas had little or no pyridoxine phosphate oxidase activity. Pyridoxine phosphate phosphatase activity was elevated in tumors relative to livers. These data indicate that the metabolism of vitamin B6 is markedly different in the hepatomas than in host or control livers and suggest that the tumor is apparently incapable of the complete synthesis of co-enzymatically active pyridoxal phosphate from inactive precursor forms such as pyridoxine.     

Polyamine concentrations in the brain of vitamin B12-deficient rats

To study the pathophysiology of the neuronal degeneration in vitamin B12 deficiency, we investigated the concentrations of the polyamines putrescine, spermidine, and spermine in brain regions and liver using high-performance liquid chromatography with fluorescence detection. Male Wistar rats were fed either a control or vitamin B12-deficient diet for 20 weeks. No remarkable behavioral changes were observed. Serum vitamin B12 and hepatic methionine concentrations were significantly lower and hepatic homocysteine was elevated in rats fed vitamin B12-deficient diet than in controls. Vitamin B12 deficiency was associated with decreased concentrations of spermidine, spermidine in liver and some regions of brain, although there were no observed abnormalities in behavior. These results suggest that vitamin B12 deficiency may play a role in neuronal degeneration through the disturbance of polyamine concentrations in rat brain.     

Effects of dietary zinc deficiency on homocysteine and folate metabolism in rats

In rats, zinc deficiency has been reported to result in elevated hepatic methionine synthase activity and alterations in folate metabolism. We investigated the effect of zinc deficiency on plasma homocysteine concentrations and the distribution of hepatic folates. Weanling male rats were fed ad libitum a zinc-sufficient control diet (382.0 nmol zinc/g diet), a low-zinc diet (7.5 nmol zinc/g diet), or a control diet pair-fed to the intake of the zinc-deficient rats. After 6 weeks, the body weights of the zinc-deficient and pair-fed control groups were lower than those of controls, and plasma zinc concentrations were lowest in the zinc-deficient group. Plasma homocysteine concentrations in the zinc-deficient group (2.3 +/- 0.2 micromol/L) were significantly lower than those in the ad libitum-fed and pair-fed control groups (6.7 +/- 0.5 and 3.2 +/- 0.4 micromol/L, respectively). Hepatic methionine synthase activity in the zinc-deficient group was higher than in the other two groups. Low mean percentage of 5-methyltetrahydrofolate in total hepatic folates and low plasma folate concentration were observed in the zinc-deficient group compared with the ad libitum-fed and pair-fed control groups. The reduced plasma homocysteine and folate concentrations and reduced percentage of hepatic 5-methyltetrahydrofolate are probably secondary to the increased activity of hepatic methionine synthase in zinc deficiency.     

Zinc and insulin metabolism

A review of experimental studies of the effect of zinc nutrition on insulin metabolism is presented. In addition to a short introduction to the synthesis, secretion, and action of insulin, the effects of zinc deficiency—specifically on glucose tolerance, insulin secretion, insulin synthesis and storage, and on total insulin-like activity—are dealt with. The concentrations of zinc and chromium in serum, pancreas, and liver are compared to those of zinc-deficient animals and pair-fed controls. In contrast to pair-fed controls, zinc-deficient rats had unaltered proinsulin contents after glucose stimulation, but they showed a diminished glucose tolerance, lowered serum insulin content, and an elevated total insulin-like activity. The serum zinc concentration of the deficient animals was greatly reduced and did not change during glucose stimulation, whereas it rose in the case of the pair-fed controls. The serum chromium concentration increased in both groups in response to glucose stimulation. In the pancreas of the deficient animals, the zinc concentration was reduced 60% and it increased during the glucose tolerance test. In the liver there were no significant differences. The chromium concentrations were elevated in both the pancreas and liver of the zinc-deficient rats by 60 and 100%, respectively, and were not influenced by glucose injection. These studies show clearly that nutritional zinc deficiency influences insulin metabolism and action.     

Factors affecting formiminoglutamic acid excretion in vitamin B12 deficiency

1. Formiminoglutamic acid, a product of the catabolism of histidine, is excreted in abnormally large amounts in the urines of vitamin B(12)-deficient rats and of vitamin B(12)-deficient sheep; the excretion is reduced to negligible amounts after administration of vitamin B(12). 2. After administration of certain methyl donors to vitamin B(12)-deficient rats or sheep urinary excretion of formiminoglutamic acid is temporarily decreased. 3. Irrespective of the pteroylglutamic acid status of the animals neither vitamin B(12)-deficient rats nor vitamin B(12)-deficient sheep have the ability to deal efficiently with histidine. 4. In sheep, urinary excretion of formiminoglutamic acid is increased after administration of aminopterin; treatment with pteroylglutamic acid restores the ability of the animal to deal with the catabolic products of histidine. 5. The possible functions of vitamin B(12) and methionine in relieving a virtual deficiency of pteroylglutamic acid are discussed.     

Metabolic effects of propionate in normal and vitamin B12-deficient rats

1. Administration of propionate caused a twofold increase in the concentrations of lactate and pyruvate in the blood of vitamin B(12)-deficient rats, whereas there was a slight decrease in lactate and a 50% increase in pyruvate in normal rats. 2. Concentrations of total ketone bodies in the blood of normal rats were not significantly altered by propionate administration but the [3-hydroxybutyrate]/[acetoacetate] ratio decreased from 3.0 to 2.0. In the vitamin B(12)-deficient rats there was a 40% decrease in total ketone bodies and a change in the ratio from 3.4 to 1.2. 3. The changes in the concentration of ketone bodies in freeze-clamped liver preparations were similar in pattern to those observed in blood. 4. Propionate administration caused a decrease in the concentration of acetyl-CoA in the livers of both groups of animals, but the absolute decrease was greater in the vitamin B(12)-deficient group. The decrease in the concentration of CoA was similar in both groups. 5. As in blood, there were threefold increases in the concentrations of lactate and pyruvate in the livers of the vitamin B(12)-deficient rats after propionate administration, whereas there was no significant change in the concentrations of these metabolites in the normal rats. 6. There was a 50% inhibition of glucose synthesis in perfused livers from vitamin B(12)-deficient rats when lactate and propionate were substrates as compared with lactate alone. 7. It is concluded that the conversion of lactate into glucose is inhibited in vitamin B(12)-deficient rats after propionate administration, and that this effect is due to inhibition of the pyruvate carboxylase step resulting from a decrease in acetyl-CoA concentration and a postulated increase in methylmalonyl-CoA concentration.     

Vitamin B6 deficiency decreases the glucose utilization in cognitive brain structures of rats

The effects of vitamin B(6) deficiency on metabolic activities of brain structures were studied. Male Sprague-Dawley weanling rats received one of the following diets: (1) 7 mg pyridoxine HCl/kg (control group); (2) 0 mg pyridoxine HCl/kg (vitamin B(6)-deficient group); or (3) 7 mg pyridoxine HCl/kg with food intake restricted in quantity to that consumed by the deficient group (pair-fed control group). After 8 weeks of dietary treatment, rats in all three groups received an intravenous injection of 2-deoxy-[(14)C] glucose (100 microCi/kg). Vitamin B(6) status was evaluated by plasma pyridoxal 5'-phosphate concentrations. The vitamin B(6)-deficient group had significantly lower levels of plasma pyridoxal 5'-phosphate than did the control and pair-fed groups. The local cerebral glucose utilization rates in structures of the limbic system, basal ganglia, sensory motor system, and hypothalamic system were determined. The local cerebral glucose utilization rates in each of the four brain regions in the deficient animals were approximately 50% lower (P < 0.05) than in the control group. Results of the present study suggest that serious cognitive deficit may occur in vitamin B(6)-deficient animals.     

Effect of oral methionine and vitamin E on blood lipid peroxidation in vitamin B6 deficient rats

Lipid peroxidation in blood of vitamin B6 deficient rats was significantly increased when compared to pair-fed controls. The observed increased lipid peroxidation in vitamin B6 deficiency was correlated with high levels of lipids, metal ions and low levels of antioxidants, alpha-tocopherol, ascorbic acid and reduced GSH. Supplementation of methionine or vitamin E along with the vitamin B6 deficient diet restored the levels of antioxidants to near normal and also protected against oxidative stress. However plasma TBARS level as well as total lipids were still elevated in M-B6 diet fed rats and normalized in E-B6-d rats.     

Vitamin B6 deficiency alters tissue iron concentrations in the Wistar rat

PurposeWe investigated the effect of a vitamin B6 deficiency and pair-feeding on tissue trace element status.MethodTissue zinc, copper and iron concentrations were measured in 3 groups of young, male Wistar rats receiving a diet of 3.5 mg/kg (control group), 0 mg/kg (deficient group) and a pair-fed group over 8 weeks. The pair-fed group received the same diet consumed by the control. Tissue trace element analysis was performed using atomic absorption spectrophotometry and plasma vitamin B6 status was determined using HPLC.ResultsDeficiency resulted in elevation in liver iron concentration and reduction in muscle iron concentration. Muscle copper concentrations were reduced in the pair-fed and deficient groups vs. the control group. Tissue zinc concentrations remained unaffected by the deficiency. Kidney iron and heart copper levels were elevated in the pair-fed group.ConclusionsThe liver and muscle iron changes were due to the deficiency and not to reduced calorie intake and the latter may be due to impaired heme synthesis. The differences in copper between the groups were due to reduced food intake. Zinc seems to form a fixed pool in these animals. A dietary deficiency of vitamin B6 impacts on the trace element status of certain tissues in key metabolic tissues and hence needs to be factored into the amelioration of the condition.     

Effects of Perinatal Vitamin B6 Deficiency on Dopaminergic Neurochemistry

Long-Evans dams were fed either a vitamin B6-deficient or a control diet from day 13-14 of gestation and throughout lactation. A control pair-fed group was also included because of differences in food intake between vitamin B6-deficient and control ad libitum dams. The progeny of vitamin B6-deficient dams had all the classic symptoms of B6 deficiency. These included weight loss, ataxia, tremor, and epileptic seizures. Concentrations of the neurotransmitter dopamine (DA), and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as D-2 dopamine receptor binding, 3,4-dihydroxyphenylalanine (DOPA) decarboxylase activity, and vitamin B6 levels were measured in the corpus striatum of progeny at 7, 14, and 18 days after birth. Striatal DA and HVA levels were significantly decreased in B6-deficient animals when compared to ad libitum or pair-fed controls. Daily injections of vitamin B6 to deprived animals from the 14th to 18th day after birth improved the abnormal movement and normalized the concentration of DA but not of HVA in corpus striatum. Striatal D-2 dopamine receptor binding using [3H]spiperone as ligand was significantly reduced in 18-day-old animals as compared to ad libitum and pair-fed controls. No significant differences were found at 14 days. The administration of vitamin B6 to deprived animals did not raise the level of D-2 receptor binding during the period of observation. Scatchard plots indicated that the differences in binding were due to changes in receptor number and not in KD. Corpus striatum DOPA decarboxylase activity with and without the addition of exogenous pyridoxal phosphate was significantly reduced in 14- and 18-day-old animals when compared to pair-fed controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Carnitine metabolism in the vitamin B-12-deficient rat.

In vitamin B-12 (cobalamin) deficiency the metabolism of propionyl-CoA and methylmalonyl-CoA are inhibited secondarily to decreased L-methylmalonyl-CoA mutase activity. Production of acylcarnitines provides a mechanism for removing acyl groups and liberating CoA under conditions of impaired acyl-CoA utilization. Carnitine metabolism was studied in the vitamin B-12-deficient rat to define the relationship between alterations in acylcarnitine generation and the development of methylmalonic aciduria. Urinary excretion of methylmalonic acid was increased 200-fold in vitamin B-12-deficient rats as compared with controls. Urinary acylcarnitine excretion was increased in the vitamin B-12-deficient animals by 70%. This increase in urinary acylcarnitine excretion correlated with the degree of metabolic impairment as measured by the urinary methylmalonic acid elimination. Urinary propionylcarnitine excretion averaged 11 nmol/day in control rats and 120 nmol/day in the vitamin B-12-deficient group. The fraction of total carnitine present as short-chain acylcarnitines in the plasma and liver of vitamin B-12-deficient rats was increased as compared with controls. When the rats were fasted for 48 h, relative or absolute increases were seen in the urine, plasma, liver and skeletal-muscle acylcarnitine content of the vitamin B-12-deficient rats as compared with controls. Thus vitamin B-12 deficiency was associated with a redistribution of carnitine towards acylcarnitines. Propionylcarnitine was a significant constituent of the acylcarnitine pool in the vitamin B-12-deficient animals. The changes in carnitine metabolism were consistent with the changes in CoA metabolism known to occur with vitamin B-12 deficiency. The vitamin B-12-deficient rat provides a model system for studying carnitine metabolism in the methylmalonic acidurias.     

Effects of nitrous oxide inactivation of vitamin B12 and of methionine on folate coenzyme metabolism in rat liver, kidney, brain, small intestine and bone marrow

The effects of nitrous oxide inactivation of the vitamin B12-dependent enzyme, methionine synthetase (EC 2.1.1.13), and of methionine on folate coenzyme metabolism were determined in rat liver, kidney, brain, small intestine and bone marrow cells. Nitrous oxide exposure led to an increase in the proportion of 5-methyltetrahydrofolate at the expense of other reduced folates in all tissues examined. Administration of methionine at levels up to 400 mg/kg resulted in the normalization of folate coenzyme patterns in liver as a result of the increased levels of S-adenosylmethionine. In other tissues examined, methionine had no effect on the levels of S-adenosylmethionine or S-adenosylhomocysteine, or on the distribution of folate coenzymes. These results are consistent with the methyl trap hypothesis as the explanation of the relationship between vitamin B12 and folate metabolism, and provide direct evidence that the sparing effect of methionine on folate metabolism is a phenomenon restricted to the liver.     

Vitamin A and isoprenoid synthesis in the rat

1. Vitamin A-deficient rats were compared with similar animals given small amounts of vitamin A sufficient for adequate growth and with animals given large amounts of vitamin A. The effects of pair-feeding and feeding ad libitum were compared. 2. Ubiquinone and cholesterol concentrations in liver were measured at various stages of the deficiency, and the uptake of radioactive mevalonate and acetate into isoprenoid compounds was studied. 3. Ubiquinone concentrations in liver increased markedly in deficient rats compared with adequate controls, and heavy vitamin A supplementation had a further effect in depressing ubiquinone concentrations. These effects were unrelated to food intake or to the size of the organs. 4. Radioactive uptake into ubiquinone was often greater in deficient livers, especially during the early stages of the experiments, but the effect was not consistent. 5. Cholesterol concentrations were usually higher in deficient livers and these were more affected by the feeding regimen. 6. No consistent effect of vitamin A deficiency or of vitamin A dosage on the incorporation of mevalonate into cholesterol or squalene was found. 7. No evidence has been found for a specific effect of vitamin A on isoprenoid synthesis at the metabolic level.     

The role of vitamin B12 in the metabolism of Euglena gracilis var. bacillaris

1. The concentrations of RNA, DNA and protein are decreased in cells of Euglena gracilis var. bacillaris grown on suboptimum concentrations of vitamin B(12). 2. The addition of vitamin B(12) to deficient cells stimulates the incorporation of [(14)C]formate into the above cell components as well as into thymine of DNA and serine and methionine of protein. 3. In a cell-free system from vitamin B(12)-deficient cells, the incorporation of labelled formate into thymidylate is decreased to a greater extent with uridine than with deoxyuridine as the substrate. 4. The addition of unlabelled glutamate dilutes the radioactivity incorporated into thymine from labelled formate. 5. These results are interpreted to mean that, in DNA synthesis, vitamin B(12) has a greater role in the reduction of ribotides to deoxyribotides than in the reduction of formate to thymine methyl and that the vitamin B(12)-dependent conversion of glutamate into beta-methylaspartate also contributes to thymine synthesis.     

Folic acid deficiency and methyl group metabolism in rat brain: effects of L-dopa

Rats deficient in folic acid were found to have decreased concentrations of S-adenosylmethione in brain, kidney, and liver. They also showed decreased concentrations of methionine in serum, but not in brain. Administration of l-dopa (a methyl acceptor) in doses comparable to those used in the treatment of Parkinson's disease caused significant reductions in the concentrations of brain methionine in rats deficient in folic acid (45%, 45 min after administration), but failed to alter methionine concentrations in control animals. The changes in brain methionine brought about by l-dopa were not paralleled by similar changes in serum methionine, which decreased by only 20%. These observations suggest that de novo methyl group synthesis contributes significantly to the maintenance of brain methionine concentrations. The possibility is raised that the daily requirements for folic acid and for vitamin B₁₂ may be increased in human patients treated chronically with large doses of l-dopa.     

Effect of thiouracil in modifying folate function in severe vitamin B12 deficiency

The effects of thiouracil in correcting defects in folic acid function produced by B12 deficiency were studied. Addition of the thyroid inhibitor, thiouracil, to a low methionine diet containing B12, increased the oxidation of [2-14C]histidine to carbon dioxide, and increased liver folate levels. Addition of 10% pectin to the diet accentuated B12 deficiency as evidenced by a greatly decreased rate of histidine oxidation (0.19%) and an increased excretion of methylmalonic acid. Addition of thiouracil to the diet restored folate function as measured by increased histidine oxidation and increased liver folate levels similar to that produced by addition of methionine to a B12-deficient diet. Thiouracil decreased methylmalonate excretion, and increased hepatic levels of B12 in animals on both B12-deficient and -supplemented diets. Hepatic methionine synthase was increased by thiouracil, which may be the result of the elevated B12 levels. S-Adenosylmethionine and the enzyme methionine adenosyltransferase were also increased by thiouracil. Thus it is possible that the effect of thiouracil in increasing folate function consists both in the effect of thiouracil in decreasing levels of methylenetetrahydrofolate reductase, and also in its action in increasing S-adenosylmethionine which exerts a feedback inhibition of this enzyme.