High concentration of be' aine-homocysteine methyltransferase in rhesus monkey lens

Purpose. A prominent polypeptide ot approximately 45kD has been observed by earlier investigators to co-purify with he βH-crystallin from rhesus monkey lens. The purpose of this study was to identify this protein and to attempt to determine its function in the lens. Methods. The βH fraction was isolated by gel exclusion jhromatography on Superose HRI2. T be 45kD band was identified by microsequencing of tryptic peptides obtained following transfer of polypeptides resolved by SDS-PAGE to immobilen. Enzyme activity was assayed as described by Garrow (J. Biol. Chem. 271:22831-38. 1996). Results. Sequences obtained from two tryptic peptides totaling 32 residues were found to be 100% identical to sequences reported for human and pig betaine-homocysteine methyltransferase (BHMT; E.G. 2.1.1.5). Using betaine is substrate, monkey lens was found to have several hundred told greater BHMT activity than lenses from rabbit, guinea pig or cow. BHMT, which produces methion ne from homocysteine using betaine as a methyl donor, is a major enzyme in li"er constituting up to 2% of total protein. The specific activity found in monkey lens is 1/3 to 1/2 that reported for liver. Conclusions. BHMT is expressed in ai active form in the lens of the rhesus monkey at levels far exceeding those leported for tissues other than liver or kidney where it ,s a major enzyme, Tte possibility that BHMT, which is involved in osmoregulation and regulation of m;thionine metabolism in other tissues, has similar functions in the lens will be di icussed as will the possibility that elevated expression of BHMT in monkey lens could be a phenomenon analogous to the recruitment of enzymes by the lens to serve as "crystallins". None.