Higher total plasma homocysteine in vitamin B12 deficiency than in heterozygosity for homocystinuria due to cystathionine beta-synthase deficiency.

Homocysteine is an amino acid considered to cause vascular injury, arteriosclerosis, and thromboembolism. Total plasma homocysteine (free and protein-bound) was found to be twice as high in asymptomatic vitamin B12-deficient subjects (23.8 +/- 3.8 mumol/L, means +/- SEM, n = 20) as in controls (11.5 +/- 0.9 mumol/L, P less than .0001, n = 21), and higher than in heterozygotes for homocystinuria due to cystathionine beta-synthase deficiency (13.8 +/- 1.6 mumol/L, P less than .01, n = 14), who were recently shown to be much more common among patients with premature vascular disease than expected. Eight (40%) vitamin B12-deficient and two (14%) heterozygote subjects had significant homocysteinemia (greater than mean +2 SD for controls). After administration of hydroxycobalamin to vitamin B12-deficient subjects, homocysteine levels decreased to normal (-49%, 12.2 +/- 1.5 mumol/L, P less than .0001, n = 20). Thus, if homocysteine does cause vascular injury, theoretically vitamin B12-deficiency might be associated with an increased frequency of vascular disease.