Glycine N-methyltransferase (GNMT), first isolated as a folate binding protein, is an abundant tetrameric enzyme in liver cytosol and the exocrine pancreas. It maintains the ratio of S- adenosylmethionine (SAM) to S-adenosylhomocysteine which regulates all methylation reactions. Folate is needed for the de novo synthesis of methyl groups. A relationship between methyl group metabolism and pancreatic exocrine secretion has been known for many years. We have previously shown that folate deficiency reduces the SAM/SAH ratio and inhibits pancreatic exocrine secretion in vivo. We also showed that treatments which elevated SAH levels in the AR42J pancreatic exocrine cell line inhibited secretion. The C-terminus of small G-proteins is carboxylmethylated in a reversible manner and we also showed that structural analogues of the G-protein C-terminus are potent inhibitors of exocrine secretion. Our first hypothesis is that elevation of SAH levels inhibits methylation of a small G protein necessary for exocrine secretion.
Specific Aim 1 is to isolate the putative methylated intermediate(s) involved in exocrine secretion and determine the effects of folate deficiency on their formation. Recent studies in another laboratory have suggested a novel non-enzymatic role for GNMT subunits as a receptor for certain polycyclic aromatic hydrocarbons in the induction of cytochrome P450. We have shown that fluorescein-labeled GNMT dissociates the tetrameric enzyme into monomers by modifying critical lysines that are used in subunit interaction. The monomeric form rapidly enters the nucleus where it binds to chromatin, probably DNA. Our second hypothesis is that a small amount of monomeric GNMT is formed from the tetrameric enzyme in the cytosol and it is the monomer which can enter the nucleus.
Specific Aim 2 is to determine the factors responsible for dissociation of tetrameric GNMT and determine to which region of DNA the dissociated GNMT is bound.
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