James D. Finkelstein

Homocystinuria: An Enzymatic Defect

A deficiency, or absence, of cystathionine synthetase activity has been demonstrated in liver obtained from a mentally retarded child with homocystinuria.

Activation of cystathionine synthase by adenosylmethionine and adenosylethionine

Summary The administration of ethionine results in a rapid and marked increase in cystathionine synthase in rat liver. The specific activity doubles within 20 minutes following a dose of 400 mg/kg. Pretreatment with either cycloheximide or actinomycin D fails to prevent this response. Preincubation of crude liver extracts with ethionine, methionine or ATP does not affect the specific activity. However, preincubation with ATP together with either methionine or ethionine leads to a marked increase in cystathionine synthase. This finding is duplicated by the preincubation of partially-purified cystathionine synthase with S-adenosylmethionine.

Methionine metabolism in mammals. Regulation of homocysteine methyltransferases in rat tissue

We studied two mammalian enzymes capable of remethylating homocysteine. Betaine-homocysteine methyltransferase was found only in rat liver while N 5 -methyltetrahydrofolate-homocysteine methyltransferase could be demonstrated in all tissues except small intestinal mucosa. Various hormones significantly affect the specific activity of the enzymes. These effects depend on hormone, enzyme, and tissue. The responses of the two transmethylases to dietary changes differed markedly. Betaine-homocysteine methyltransferase increased with protein and methionine feeding. N 5 -Methyltetrahydrofolate-homocysteine methyltransferase on the other hand increased under conditions that suggested the need for methionine synthesis. We conclude that N 5 -methyltetrahydrofolate-homocysteine methyltransferase activity contributes significantly to the regulation of methionine metabolism in mammals. High protein diets repress the synthesis of this enzyme in liver.

Homocystinuria associated with decreased methylenetetrahydrofolate reductase activity

Summary A new type of homocystinuria is described. A variety of evidence indicates that patients with this type of homocystinuria are not deficient in cystathionine synthase activity. Fibroblasts from these patients were unable to grow as rapidly as control fibroblasts if homocystine replaced methionine in the culture medium. N 5 -Methyltetrahydrofolate-homocysteine methyltransferase activity in these cells was not markedly decreased, whereas methylenetetrahydrofolate reductase activity was significantly below normal. A deficiency of this reductase activity can explain the biochemical abnormalities in these patients.

S-adenosylhomocysteine hydrolase deficiency in a human: A genetic disorder of methionine metabolism

We report studies of a Croatian boy, a proven case of human S-adenosylhomocysteine (AdoHcy) hydrolase deficiency. Psychomotor development was slow until his fifth month; thereafter, virtually absent until treatment was started. He had marked hypotonia with elevated serum creatine kinase and transaminases, prolonged prothrombin time and low albumin. Electron microscopy of muscle showed numerous abnormal myelin figures; liver biopsy showed mild hepatitis with sparse rough endoplasmic reticulum. Brain MRI at 12.7 months revealed white matter atrophy and abnormally slow myelination. Hypermethioninemia was present in the initial metabolic study at age 8 months, and persisted (up to 784 μM) without tyrosine elevation. Plasma total homocysteine was very slightly elevated for an infant to 14.5–15.9 μM. In plasma, S-adenosylmethionine was 30-fold and AdoHcy 150-fold elevated. Activity of AdoHcy hydrolase was ≈3% of control in liver and was 5–10% of the control values in red blood cells and cultured fibroblasts. We found no evidence of a soluble inhibitor of the enzyme in extracts of the patients cultured fibroblasts. Additional pretreatment abnormalities in plasma included low concentrations of phosphatidylcholine and choline, with elevations of guanidinoacetate, betaine, dimethylglycine, and cystathionine. Leukocyte DNA was hypermethylated. Gene analysis revealed two mutations in exon 4: a maternally derived stop codon, and a paternally derived missense mutation. We discuss reasons for biochemical abnormalities and pathophysiological aspects of AdoHcy hydrolase deficiency.

Glycine N-methyltransferase deficiency: A novel inborn error causing persistent isolated hypermethioninaemia

This paper reports clinical and metabolic studies of two Italian siblings with a novel form of persistent isolated hypermethioninaemia, i.e. abnormally elevated plasma methionine that lasted beyond the first months of life and is not due to cystathionine β-synthase deficiency, tyrosinaemia I or liver disease. Abnormal elevations of their plasma S-adenosylmethionine (AdoMet) concentrations proved they do not have deficient activity of methionine adenosyltransferase I/III. A variety of studies provided evidence that the elevations of methionine and AdoMet are not caused by defects in the methionine transamination pathway, deficient activity of methionine adenosyltransferase II, a mutation in methylenetetrahydrofolate reductase rendering this activity resistant to inhibition by AdoMet, or deficient activity of guanidinoacetate methyltransferase. Plasma sarcosine (N-methylglycine) is elevated, together with elevated plasma AdoMet in normal subjects following oral methionine loads and in association with increased plasma levels of both methionine and AdoMet in cystathionine β-synthase-deficient individuals. However, plasma sarcosine is not elevated in these siblings. The latter result provides evidence they are deficient in activity of glycine N-methyltransferase (GNMT). The only clinical abnormalities in these siblings are mild hepatomegaly and chronic elevation of serum transaminases not attributable to conventional causes of liver disease. A possible causative connection between GNMT deficiency and these hepatitis-like manifestations is discussed. Further studies are required to evaluate whether dietary methionine restriction will be useful in this situation.

Ethanol-induced changes in methionine metabolism in rat liver

Abstract The administration of alcohol to rats fed a protein-restricted diet results in significant changes in the hepatic content of four enzymes of methionine metabolism. The levels of s-adenosylmethionine synthetase, cystathionine synthase, and betaine-homocysteine methyltransferase increase while the level of methyltetrahydrofolate-homocysteine methyltransferase decreases. These changes represent a reversal of the normal adaptive response to protein-restriction. The resultant impairment in methionine conservation could explain the alcohol-induced increase in the dietary lipotrope requirement.

Methionine metabolism in mammals: Kinetic study of betaine-homocysteine methyltransferase

Abstract We have studied the kinetic properties of betaine-homocysteine methyltransferase prepared from rat liver. The Michaelis constants for the substrates are K hetaine = 48 μm and K homocysteine = 12 μm . The reaction conforms to the Ordered Bi Bi model. Homocysteine is the first substrate to add to the enzyme and N,N -dimethylglycine is the first product released. In addition to the reaction products, l -cysteine and l -cystine also inhibit the enzyme. These effects of methionine and cyst(e)ine on the betaine-chomocysteine methyltransferase may be significant in the regulation of methionine metabolism in the intact animal.

Methionine metabolism in mammals: Regulatory effects of S-adenosylhomocysteine

Abstract S -Adenosylhomocysteine inhibits betaine-homocysteine methyltransferase. The inhibition is nonlinear, competitive in relation to homocysteine, and noncompetitive in relation to betaine. S -Adenosylhomocysteine activates cystathionine synthase at all concentrations of the substrates, serine and homocysteine. By altering the distribution of homocysteine between these competing pathways, S -adenosylhomocysteine may be significant in the regulation of methionine metabolism in the intact animal.

Homocystinuria due to Cystathionine Synthetase Deficiency: The Mode of Inheritance

Deficiency of cystathioninie synthetase activity results in the clinical syndrome of homocystinuria. In both parents of a patient with homocystinuria, the hepatic cystathionine synthetase activity was 40 percent of that in unrelated control patients. These findings demonstrate that the metabolic error is inherited and suggest that the parents, although clinically normal, represent the heterozygous. state. A second case of homocystinuria also is shown to be associated with cystathionine synthetase deficiency.