Betaine:homocysteine methyltransferase--a new assay for the liver enzyme and its absence from human skin fibroblasts and peripheral blood lymphocytes.
Chronic elevation of plasma homocysteine is associated with increased atherogenesis and thrombosis, and can be lowered by betaine (N,N,N-trimethylglycine) treatment which is thought to stimulate activity of the enzyme betaine:homocysteine methyltransferase. We have developed a new assay for this enzyme, in which the products of the enzyme-catalysed reaction between betaine and homocysteine are oxidised by performic acid before being separated and quantified by amino acid analysis. This assay confirmed that human liver contains abundant betaine:homocysteine methyltransferase (33.4 nmol/h/mg protein at 37 degrees C, pH 7.4). Chicken and lamb livers also contain the enzyme, with respective activities of 50.4 and 6.2 nmol/h/mg protein. However, phytohaemagglutinin-stimulated human peripheral blood lymphocytes and cultured human skin fibroblasts contained no detectable betaine:homocysteine methyltransferase (less than 1.4 nmol/h/mg protein), even after cells were pre-cultured in media designed to stimulate production of the enzyme. The results emphasize the importance of the liver in mediating the lowering of elevated circulating homocysteine by betaine.
Wang
J A
JA
Department of Cardiovascular Medicine, Prince Henry Hospital, University of New South Wales, Sydney, Australia.
Dudman
N P
NP
Lynch
J
J
Wilcken
D E
DE
D000818:Animals; D001622:Betaine / Q000494:pharmacology; D050940:Betaine-Homocysteine S-Methyltransferase; D002645:Chickens; D005347:Fibroblasts / Q000201:enzymology*; D006710:Homocysteine / Q000097:blood / Q000302:isolation & purification; D006801:Humans; D008099:Liver / Q000201:enzymology*; D008214:Lymphocytes / Q000201:enzymology*; D008715:Methionine / Q000302:isolation & purification; D008780:Methyltransferases / Q000032:analysis* / Q000378:metabolism; D012756:Sheep; D012867:Skin / Q000201:enzymology*
D006710:Homocysteine; D001622:Betaine; D008715:Methionine; D008780:Methyltransferases; C494548:BHMT protein, human; D050940:Betaine-Homocysteine S-Methyltransferase