Characterization of homocysteine metabolism in the rat liver

Recent evidence suggests that an increased plasma concentration of the sulphur amino acid homocysteine is a risk factor for the development of vascular disease. The tissue(s) responsible for homocysteine production and export to the plasma are not well known. However, given the central role of the liver in amino acid metabolism, we developed a rat primary hepatocyte model in which homocysteine (and cysteine) production and export were examined. The dependence of homocysteine export from incubated hepatocytes on methionine concentration fitted well to a rectangular hyperbola, with half-maximal homocysteine export achieved at methionine concentrations of approx. 0.44 mM. Hepatocytes incubated with 1 mM methionine and 1 mM serine (a substrate for the transulphuration pathway of homocysteine removal) produced and exported significantly less homocysteine (25-40%,) compared with cells incubated with 1 mM methionine alone. The effects of dietary protein on homocysteine metabolism were also examined. Rats fed a 60% protein diet had a significantly increased total plasma homocysteine level compared with rats fed a 20% protein diet. In vitro effects of dietary protein were examined using hepatocytes isolated from animals maintained on these diets. When incubated with 1 mM methionine, hepatocytes from rats fed the high protein diet exported significantly more homocysteine compared with hepatocytes from rats fed the normal protein diet. Inclusion of serine significantly lowered homocysteine export in the normal protein group, but the effect was more marked in the high protein group. In vivo effects of serine were also examined. Rats fed a high protein diet enriched with serine had significantly lower total plasma homocysteine (25-30%) compared with controls. These data indicate a significant role for the liver in the regulation of plasma homocysteine levels.