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Riboflavin is linked to the metabolism of B-vitamins involved in homocysteine remethylation and transsulfuration. Riboflavin is transformed to FMN and FAD by the action of riboflavin kinase (EC 2.7.1.26) and FAD synthase (EC 2.7.7.2.). FMN and FAD are cofactors for vitamin B6, cobalamin and folate metabolizing enzymes. Vitamin B6 and cobalamin serve as cofactors for enzymes involved in homocysteine metabolism, while folate serves as a one-carbon donor for the remethylation of homocysteine. Abbreviations: Cbl(I), cob(I)alamin; Cbl(II), cob(II)alamin; CBS, cystathionine β-synthase; CH2-THF, 5,10-methylenetetrahydrofolate; CH3-THF, 5-methyltetrahydrofolate; CL, cystathionine lyase; Cys, cysteine; FS, FAD synthase; Met, methionine; MS, methionine synthase; MSR, methionine synthase reductase; MTHFR, 5,10-methylenetetrahydrofolate reductase; PLP, pyridoxal-5'-phosphate; PMP, pyridoxamine-5'-phosphate; PNP, pyridoxine-5'-phosphate; PPO, pyridoxine-5'-phosphate oxidase; RK, riboflavin kinase; THF, tetrahydrofolate.

Dose-response curves for the relationship between plasma riboflavin and plasma homocysteine according to MTHFR 677C→T genotype. Solid lines indicate estimated dose-response curves, and shaded areas represent 95% confidence intervals. The curves are obtained by additive Gaussian regression analysis, and the model is adjusted for sex, age, serum folate, serum cobalamin, and serum creatinine. The figure is modified from reference 22 with permission.