Transsulfuration in an adult with hepatic methionine adenosyltransferase deficiency.

We investigated sulfur and methyl group metabolism in a 31-yr-old man with partial hepatic methionine adenosyltransferase (MAT) deficiency. The patient's cultured fibroblasts and erythrocytes had normal MAT activity. Hepatic S-adenosylmethionine (SAM) was slightly decreased. This clinically normal individual lives with a 20-30-fold elevation of plasma methionine (0.72 mM). He excretes in his urine methionine and L-methionine-d-sulfoxide (2.7 mmol/d), a mixed disulfide of methanethiol and a thiol bound to an unidentified group X, which we abbreviate CH3S-SX (2.1 mmol/d), and smaller quantities of 4-methylthio-2-oxobutyrate and 3-methylthiopropionate. His breath contains 17-fold normal concentrations of dimethylsulfide. He converts only 6-7 mmol/d of methionine sulfur to inorganic sulfate. This abnormally low rate is due not to a decreased flux through the primarily defective enzyme, MAT, since SAM is produced at an essentially normal rate of 18 mmol/d, but rather to a rate of homocysteine methylation which is abnormally high in the face of the very elevated methionine concentrations demonstrated in this patient. These findings support the view that SAM (which is marginally low in this patient) is an important regulator that helps to determine the partitioning of homocysteine between degradation via cystathionine and conservation by reformation of methionine. In addition, these studies demonstrate that the methionine transamination pathway operates in the presence of an elevated body load of that amino acid in human beings, but is not sufficient to maintain methionine levels in a normal range.

Reduced acetylation of procainamide by para-aminobenzoic acid.

Acetylation is the major route of metabolism of many drugs including the antiarrhythmic agent procainamide. Coadministration of para-aminobenzoic acid was observed to decrease the biotransformation of procainamide to N-acetylprocainamide in a patient with rapid acetylation kinetics. In view of the distinct antiarrhythmic and toxic properties of procainamide and N-acetylprocainamide, the observed drug interference may have great clinical relevance in long-term oral antiarrhythmic therapy and in instances where other drugs converge for acetylation.

Homocysteine.

Homocysteine does not occur in the diet but it is an essential intermediate in normal mammalian metabolism of methionine. Each compound, methionine or homocysteine, is the precursor of the other. Similarly, the synthesis of one is the mechanism for the detoxification of the other. The ubiquitous methionine cycle is the metabolic basis for this relationship. In some tissues the transsulfuration pathway diverts homocysteine from the cycle and provides a means for the synthesis of cysteine and its derivatives. Methionine, (or homocysteine) metabolism is regulated by the disposition of homocysteine between these competing sequences. Both pathways require vitamin-derived cofactors, pyridoxine for transsulfuration and both folate and cobalamin in the methionine cycle. The clinical consequences of disruption of these pathways was apparent first in rare inborn errors of metabolism that cause homocystinuria, but recent studies focus on "hyperhomocysteinemia"--a lesser metabolic impairment that may result from genetic variations, acquired pathology, toxicity and nutritional inadequacy. Hyperhomocysteinemia is an independent risk factor for thrombovascular diseases however it is not clear whether the minimally increased concentration of the amino acid is the causative agent or merely a marker for the pathology. Until we resolve that question we cannot predict the potential efficacy of therapies based on folate administration with or without additional cobalamin and pyridoxine.

Homocysteine: a history in progress.

This paper shows that the linkage between basic science and clinical research has characterized the field of sulfur amino acid metabolism since 1810, when Wollaston isolated cystine from a human bladder stone. The nature and consequences of this relationship are discussed.

Maternal hyperhomocysteinemia: a risk factor for neural-tube defects?

The maternal vitamin status, especially of folate, is involved in the pathogenesis of neural-tube defects (NTDs). Maternal folate administration can prevent these malformations. The precise metabolic mechanism of the beneficial effect of folate is unclear. In this study we focus on homocysteine accumulation, which may derive from abnormalities of metabolism of folate, vitamin B12, and vitamin B6. We studied nonpregnant women, 41 of whom had given birth to infants with NTDs and 50 control women who previously had normal offspring. The determinations included the plasma total homocysteine both in the fasting state and 6 hours after the ingestion of a methionine load. In addition, we measured the fasting blood levels of folate, vitamin B12, and vitamin B6. The mean values for both basal homocysteine and homocysteine following a methionine load were significantly increased in the group of women who previously had infants with NTDs. In nine of these subjects and two controls, the values after methionine ingestion exceeded the mean control by more than 2 standard deviations. Cystathionine synthase levels in skin fibroblasts derived from these methionine-intolerant women were within the normal range. Our findings suggest a disorder in the remethylation of homocysteine to methionine due to an acquired (ie, nutritional) or inherited derangement of folate or vitamin B12 metabolism. Increased homocysteine levels can be normalized by administration of vitamin B6 or folate. Therefore, we suggest that the prevention of NTDs by periconceptional folate administration may effectively correct a mild to moderate hyperhomocysteinemia.

Pathways and regulation of homocysteine metabolism in mammals.

Two intersecting pathways, the methionine cycle and the transsulfuration sequence, compose the mechanisms for homocysteine metabolism in mammals. The methionine cycle occurs in all tissues and provides for the remethylation of homocysteine, which conserves methionine. In addition, the cycle is essential for the recycling of methyltetrahydrofolate. The synthesis of cystathionine is the first reaction in the irreversible pathway for the catabolism of homocysteine by means of the sequential conversion to cysteine and sulfate. This pathway has a limited distribution and is found primarily in the liver, kidney, small intestine and pancreas. Regulation of homocysteine metabolism is achieved by changes in the quantity of homocysteine distributed between the two competing pathways. Two mechanisms are basic to the regulatory process. Changes in tissue content of the relevant enzymes are the response to sustained perturbations. The inherent kinetic properties of the enzymes provide an immediate response to alterations in the tissue concentrations of substrates and other metabolic effectors. S-adenosylmethionine, S-adenosylhomocysteine, and methyltetrahydrofolate are of particular importance in that context.

The metabolism of homocysteine: pathways and regulation.

Two pathways, the methionine cycle and transsulfuration, account for virtually all methionine metabolism in mammals. Every tissue possesses the methionine cycle. Therefore, each can synthesize AdoMet, employ it for transmethylation, hydrolyze AdoHcy, and remethylate homocysteine. Transsulfuration, which occurs only in liver, kidney, small intestine and pancreas, is the means for catabolizing homocysteine. Liver has a unique isoenzyme of MAT that allows the utilization of excess methionine for the continued synthesis of AdoMet. Metabolic regulation is based on the distribution of available homocysteine between remethylation and conversion to cystathionine. The tissue content of the enzymes and their inherent kinetic properties provide the basis for the regulatory mechanism. The effector properties of the metabolites AdoMet, AdoHcy and methylTHF are of particular relevance.

Methionine metabolism in mammals. Adaptation to methionine excess.

We conducted a systematic evaluation of the effects of increasing levels of dietary methionine on the metabolites and enzymes of methionine metabolism in rat liver. Significant decreases in hepatic concentrations of betaine and serine occurred when the dietary methionine was raised from 0.3 to 1.0%. We observed increased concentrations of S-adenosylhomocysteine in livers of rats fed 1.5% methionine and of S-adenosylmethionine and methionine only when the diet contained 3.0% methionine. Methionine supplementation resulted in decreased hepatic levels of methyltetrahydrofolate-homocysteine methyltransferase and increased levels of methionine adenosyltransferase, betaine-homocysteine methyltransferase, and cystathionine synthase. We used these data to simulate the regulatory locus formed by the enzymes which metabolize homocysteine in livers of rats fed 0.3% methionine, 1.5% methionine, and 3.0% methionine. In comparison to the model for the 0.3% methionine diet group, the model for the 3.0% methionine animals demonstrates a 12-fold increase in the synthesis of cystathionine, a 150% increase in flow through the betaine reaction, and a 550% increase in total metabolism of homocysteine. The concentrations of substrates and other metabolites are significant determinants of this apparent adaptation.

Inactivation of betaine-homocysteine methyltransferase by adenosylmethionine and adenosylethionine.

Preincubation of betaine-homocysteine methyltransferase, prepared from rat liver, with either S-adenosylmethionine or S-adenosylethionine results in a marked loss of enzyme activity. Gel filtration did not restore activity. However both S-adenosylhomocysteine and L-homocysteine, when added to the preincubation medium, inhibited the inactivation of betaine-homocysteine methyltransferase.

Methionine metabolism in mammals. Distribution of homocysteine between competing pathways.

Using an in vitro system which contained enzymes, substrates, and other reactants at concentrations which approximated the in vivo conditions in rat liver, we measured the simultaneous product formation by three enzymes which utilize homocysteine. In the control system, 5-methyltetrahydrofolate homocysteine methyltransferase, betaine homocysteine methyltransferase, and cystathionine beta-synthase accounted for 27, 27, and 46%, respectively, of the homocysteine consumed. Subsequent studies demonstrated that the adaptation from a high protein diet to a low protein diet is achieved by a significant increase in betaine homocysteine methyltransferase, and 83% reduction in cystathionine synthase, and a total decrease of 55% in the consumption of homocysteine. S-Adenosylmethionine, by activating cystathionine synthase, contributes significantly to the regulation of the pathway.

Methionine metabolism in mammals. The methionine-sparing effect of cystine.

Cystine can replace approximately 70% of the dietary requirement for methionine. We used standard enzyme assays, determinations of the hepatic concentrations of metabolites and an in vitro system which simulates the regulatory site formed by the enzymes which utilize homocysteine in this study of the mechanism for this adaptation. A significant alteration in the pattern of hepatic homocysteine metabolism occurs following the substitution of cystine for methionine. The major change is a marked reduction in the synthesis of cystathionine. Decreases in both the level of cystathionine synthase and in the concentration of adenosyl-methionine, a positive effector of the enzyme, explain this finding. Despite significant increases in the hepatic levels of betaine-homocysteine methyltransferase and methyltetrahydrofolate-homocysteine methyltransferase, flow through these reactions remains relatively constant. The betaine enzyme may be essential for efficient methionine conservation. In the absence of choline, cystine cannot replace methionine in an adequate diet limited in the latter amino acid.

Effect of dietary cystine on methionine metabolism in rat liver.

Cystine supplementation of adequate diets resulted in significantly higher hepatic levels of betaine-homocysteine methyltransferase. Other changes occurred but were a function of the basal diet. When the latter contained 0.25% methionine + 0.5% cystine, the additional cystine caused a markedly lower hepatic cystathionine synthase activity and lower levels of both adenosylmethionine and serine. The metabolic effect of these changes may be enhanced methionine retention and diminished transsulfuration.

Effect of nicotinamide on methionine metabolism in rat liver.

To test the response to increased utilization of methyl groups, we administered large dosages of nicotinamide to rats fed an adequate diet that contained limited amounts of methionine and choline. During the 4 d after the injection, we observed several significant effects on the hepatic concentrations of the enzymes and metabolites of methionine metabolism. Methionine and S-adenosylmethionine remained at control levels; the concentrations of S-adenosylhomocysteine exceeded the control values from 4 to 16 h; and the levels of serine and betaine were lower after 16 h. Treatment with nicotinamide resulted in higher hepatic levels of methionine adenosyltransferase (after 4 h) and cystathionine synthase (after 16 h). These data indicate that increases in both homocysteine methylation and S-adenosylmethionine synthesis may be components of the response to excessive methyl group consumption. An increased synthesis of cystathionine would provide for the removal of S-adenosylhomocysteine (and homocysteine) derived from the adenosylmethionine-dependent methylation of nicotinamide.

Folate-responsive homocystinuria and "schizophrenia". A defect in methylation due to deficient 5,10-methylenetetrahydrofolate reductase activity.

Homocystinuria and homocystinemia without hypermthioninemia, but with reccurent episodes of folate responseive schizophrenic-like behavior, was documented in a mildly retarded adolescent girl who lacked the habitus associated with cystathionine synthase deficiency. Enzymes involved in homocysteine-methionine metabolism were demonstrated to be normal. A defect in the ability to reducte N-5-10--methylenetetrahydrofolate to 5-methyltetrahydrofolate was demonstrated. Methylenetetrahydrofolate reductase was 18 per cent of control values. Methyltetrahydrofolate is used for the methylation of homocysteine to methionine, and a deficiency of this compound could explain the homocystinemia and homocystinuria.