Homocysteine catabolism: levels of 3 enzymes in cultured human vascular endothelium and their relevance to vascular disease

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Elevated plasma homocysteine enhances the risk of thrombosis and premature arteriosclerosis. We have assessed the activity of the 3 prime enzymes of homocysteine metabolism in cultured human venous endothelial cells, in a study of their possible protective roles. In cells from 4 individuals, cultured in Dulbecco's modified Eagle medium, the mean activity ± S.D. of cystathionine β-synthase (nmol of product/h per mg of cell protein, at 37°C) was 3.58 ± 3.11 at pH 8.6. The assay used was our newly developed amino acid analyser-based procedure. The activity of 5-methyltetrahydrofolate:homocysteine methyltransferase at pH 7.4 was 4.12 ± 1.25 and betaine:homocysteine methyltransferase (BHMT) was undetectable (< 1.4 nmol/h per mg protein). Cells were also cultured in a medium aimed at stimulating methionine biosynthesis, containing methionine-deficient Dulbecco's modified Eagle medium to which L-homocystine (100 μmol/1) and methylcobalamin (1 μmol/1) had been added. In these cells 5-methyltetrahydrofolate:homocysteine methyltransferase activity increased to 7.95 ± 1.45, P < 0.001, there was a non-significant decrease in cystathionine β-synthase activity to 2.16 ± 1.52 and BHMT activity was still undetectable. These cells were more resistant to in vitro homocysteine-induced detachment than were cells from the same line cultured in Dulbecco's modified Eagle medium alone. Our findings establish that human endothelial cells express 2 of the 3 primary enzymes of homocysteine catabolism. They suggest that persons who are deficient in cystathionine β-synthase or 5-methyltetrahydrofolate:homocysteine methyltransferase activity may not only develop homocysteinemia, but also have vascular endothelium which is more susceptible to damage by homocysteine than persons with normal enzyme levels.