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

Abstract

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.