| Betaine-homocysteine methyltransferase (BHMT) is a critical conduit between choline oxidation and the methionine cycle. Betaine, an important methyl donor in both pathways, uses BHMT to catalyze the transfer of a methyl group from betaine to homocysteine to produce dimethylglycine and methionine, respectively. Betaine also functions as a major osmoprotectant and accumulates in the kidney under high extracellular osmolarity. Increased transport of betaine into renal cells has been studied, but the metabolic partitioning of betaine between its use as a methyl donor and its use as an osmoprotectant has not.; BHMT is the only enzyme known to metabolize betaine, leading to the hypothesis that BHMT is down-regulated under conditions requiring the accumulation of betaine, such as high salt intake. BHMT is expressed in the livers of mammals and only a few species such as pig, guinea pig, and primates (including humans) have been shown to express BHMT in the kidney. Immunodetection methods were used to demonstrate that humans and guinea pigs have similar patterns of hepatic and renal BHMT expression and suggest that the guinea pig is a suitable in vivo model for studying renal BHMT. Guinea pig, however, also demonstrated BHMT expression in the pancreas. To investigate the possible osmotic regulation of BHMT, guinea pigs were subjected to high salt intake and BHMT expression analyzed. BHMT activity in liver and kidney cortex decreased approximately 55%, but no change in pancreatic BHMT activity was observed. Neither a soluble effector nor an increase in ionic strength had an effect on BHMT activity. Choline dehydrogenase, the enzyme responsible for the committed step in de novo synthesis of betaine, showed no change in liver or kidney cortex activity with treatment. Western blot analysis indicated a 30% reduction in steady-state liver and kidney cortex BHMT protein. Real-time PCR using TagMan chemistry demonstrated a 93% reduction in steady-state liver BHMT mRNA and a 72% reduction in steady-state kidney cortex BHMT mRNA. The discrepancy in the reduction of BHMT activity, steady-state protein, and steady-state mRNA may suggest complex regulation of BHMT expression including variable turnover rates of protein and mRNA and post-translational modifications of the BHMT enzyme. |
