Plasma homocysteine levels rise with age in both men and women. This may be secondary to decreases in vitamin cofactor levels or coexisting renal impairment, which are common in this age group. Homocysteine concentrations are higher in men than women. This may be due to differences in muscle mass, renal function, essential vitamin cofactors, or even protein intake. Sex hormones may also influence homocysteine metabolism, but this remains speculative at the current time.
Homocysteine concentrations rise with falling renal function.
This increase is likely to be due to impaired metabolism of homocysteine by the kidney or by the liver, whose normal function may be perturbed by the uremic milieu. High plasma homocysteine concentrations have also been observed in patients with psoriasis and acute lymphoblastic leukemia probably reflecting high cell turnover in these disorders. The classical disorder, characterized by complete, or almost complete, absence of cystathionine b-synthase, is rare and occurs in the United States once in 400,000 births. The condition is transmitted in autosomal recessive fashion. Reduced enzyme activity is observed in heterozygotes for the disorder and may be demonstrated by administering a methionine challenge.