The atherogenic effect of excess methionine intake

Abstract

Methionine is the precursor of homocysteine, a sulfur amino acid intermediate in the methylation and transsulfuration pathways. Elevated plasma homocysteine (hyperhomocysteinemia) is associated with occlusive vascular disease. Whether homocysteine per se or a coincident metabolic abnormality causes vascular disease is still an open question. Animals with genetic hyperhomocysteinemia have so far not displayed atheromatous lesions. However, when methionine-rich diets are used to induce hyperhomocysteinemia, vascular pathology is often observed. Such studies have not distinguished the effects of excess dietary methionine from those of hyperhomocysteinemia. We fed apolipoprotein E-deficient mice with experimental diets designed to achieve three conditions: (i) high methionine intake with normal blood homocysteine; (ii) high methionine intake with B vitamin deficiency and hyperhomocysteinemia; and (iii) normal methionine intake with B vitamin deficiency and hyperhomocysteinemia. Mice fed methionine-rich diets had significant atheromatous pathology in the aortic arch even with normal plasma homocysteine levels, whereas mice fed B vitamin-deficient diets developed severe hyperhomocysteinemia without any increase in vascular pathology. Our findings suggest that moderate increases in methionine intake are atherogenic in susceptible mice. Although homocysteine may contribute to the effect of methionine, high plasma homocysteine was not independently atherogenic in this model. Some product of excess methionine metabolism rather than high plasma homocysteine per se may underlie the association of homocysteine with vascular disease.

Fig. 1.

Pathways of homocysteine and methionine metabolism. In cells, homocysteine is derived from methionine after its utilization as a methyl group donor in biological methylation reactions. In this cycle, methionine is activated by condensation with ATP to give the ubiquitous methyl donor, SAM. SAM is transformed into S-adenosylhomocysteine (SAH) by donating its methyl group to the substrates of methylation reactions. Subsequently, SAH gives rise to homocysteine in a reversible reaction that favors SAH over homocysteine production. Because SAH is a potent inhibitor of most methyltransferase enzymes, homocysteine must be constantly removed through the folate- and vitamin B₁₂-dependent remethylation of homocysteine to methionine or by the irreversible vitamin B₆-dependent degradation of homocysteine through the transsulfuration pathway. When intracellular homocysteine accumulates, excess homocysteine can be exported into circulation for clearance by kidney and liver. Methionine enters the cycle primarily from diet, but it can also be salvaged from endogenous protein degradation. Methionine can be removed from the cycle for use in protein and polyamine synthesis, or by homocysteine and the transsulfuration pathway. SAM acts as a coregulator of the methylation and transsulfuration pathways by stimulating cystathionine β-synthetase, the first step in the removal of homocysteine and inhibiting methylene tetrahydrofolate reductase, which generates the folate coenzyme used in the synthesis of methionine from homocysteine (67). Saturation of the transsulfuration pathway by excess methionine and B₆ deficiency, or by inhibition of homocysteine remethylation due to folate or B₁₂ deficiency, can lead to the production of intracellular homocysteine, which is then exported from cells into plasma. B₆, vitamin B₆, pyridoxal 5′-phosphate; B₁₂, vitamin B₁₂, methylcobalamin; THF, tetrahydrofolate.

Fig. 2.

The effect of the four dietary regimens on the aortic plaque area. Fourteen-week-old ApoE-deficient mice fed control diets develop spontaneous lesions as shown by the white bar. The lesion area increased significantly in mice fed methionine-enriched diets (light and dark gray) in comparison with controls. The highest lesion area was attained in mice that were fed the high-methionine, vitamin-deficient M+B- diet, with a mean lesion area that was nearly twice that seen in the control group (dark gray). The mean aortic lesion area in mice fed the vitamin-deficient normal-methionine B- diet was not significantly different from the mean basal lesion area in controls (white vs. black), despite the fact that the B- group had the highest plasma homocysteine levels (see Table 2). In contrast, the lesion area was significantly higher in mice fed the high-methionine, vitamin-supplemented “M+B+” diet than in controls, despite the fact that homocysteine levels in this group were normal (Table 2). Error bars represent standard errors. a, b, and c are significantly different; P < 0.05.

Fig. 3.

Ateromatous plaques in the aortic arch of ApoE-deficient mice. Hematoxylin/eosin staining of the aortic arch of an ApoE-null mouse fed the B vitamin-deficient diet (B-, Upper) and the methionine-rich and B vitamin-deficient diet (M+B-, Lower). Although both lesions (P) are classified as fatty streak (initial) lesions, lesions in the M+B- group were significantly larger. L, lumen; M, media; P, plaque. (Scale bar = 100 μm.)