Involved in cobalamin metabolism and trafficking (PubMed:18385497, PubMed:23415655, PubMed:24722857, PubMed:26364851). Plays a role in regulating the biosynthesis and the proportion of two coenzymes, methylcob(III)alamin (MeCbl) and 5'-deoxyadenosylcobalamin (AdoCbl) (PubMed:18385497, PubMed:23415655, PubMed:24722857). Promotes oxidation of cob(II)alamin bound to MMACHC (PubMed:26364851). The processing of cobalamin in the cytosol occurs in a multiprotein complex composed of at least MMACHC, MMADHC, MTRR (methionine synthase reductase) and MTR (methionine synthase) which may contribute to shuttle safely and efficiently cobalamin towards MTR in order to produce methionine (PubMed:27771510). {ECO:0000269|PubMed:18385497, ECO:0000269|PubMed:23415655, ECO:0000269|PubMed:24722857, ECO:0000269|PubMed:26364851, ECO:0000269|PubMed:27771510}. This is the function of Ensembl gene identifier ENSG00000168288 (MMADHC, metabolism of cobalamin associated D).