Date of Award
Doctor of Philosophy
Food Science and Human Nutrition
Kevin L. Schalinske
Glycine N-methyltransferase (GNMT) regulates S-adenosylmethionine (SAM) levels and the SAM/S-adenosylhomocysteine (SAH) ratio. Therefore, up-regulation of GNMT may lead to wastage of methyl groups required for transmethylation reactions. Previously, we demonstrated 13-cis-retinoic acid (CRA) reduced hepatic SAM concentrations and the SAM/SAH ratio in rats. In Study 1 the ability of CRA, as well as all-trans-retinoic acid (ATRA), to regulate hepatic GNMT were examined as a potential basis for our observations. Rats were fed either a control or a methionine-supplemented (MS) diet and given either ATRA, CRA (30 mumol/kg BW), or vehicle daily for 7 d. In control rats, both CRA and ATRA elevated hepatic GNMT activity (49 and 34%, respectively), compared to the control group. Similar results were observed in MS rats. Moreover, enzymatic activity was reflected in GNMT protein abunDance; The second study determined that vitamin A had a similar ability to up-regulate GNMT. Additionally, GNMT induction down-regulated the transmethylation pathway. Rats were given retinyl palmitate (RP), CRA, ATRA, or vehicle daily for 10 d. RP, CRA, and ATRA elevated hepatic GNMT activity 32, 74, and 124% respectively, compared to the control group. Moreover, the retinoid-mediated changes in GNMT activity were reflected in GNMT abundance (38, 89, and 107% increase for RP-, CRA-, and ATRA-treated rats, respectively). In addition, hepatic DNA was hypomethylated (~100%) following ATRA treatment compared to the control group, whereas the transsulfuration product glutathione was unaffected. Because retinoids stimulate gluconeogenesis, the third study was undertaken to determine the relationship between ATRA and gluconeogenic hormones on these metabolic pathways. Adrenal function was not required for ATRA to induce and activate hepatic GNMT; however, treatment of rats with dexamethasone (1 mg/kg BW, daily for 5 d) induced hepatic GNMT (~2-fold). Increased plasma total homocysteine levels observed in adrenalectomized rats were normalized by either ATRA or DEX, indicating that homocysteine catabolism was enhanced. Moreover, co-administration of ATRA and DEX resulted in an additive effect on hepatic GNMT induction in both rats and rat hepatoma cells. Collectively, these results demonstrate that both ATRA and DEX alter methyl group metabolism, thereby having implications for an interaction between retinoid administration and diabetes.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Matthew James Rowling
Rowling, Matthew James, "Nutritional and hormonal modulation of glycine N-methyltransferase: implications for aberrant methyl group metabolism " (2004). Retrospective Theses and Dissertations. 814.