| Summary: | Thesis (M.Sc.)--Memorial University of Newfoundland, 2009. Biochemistry Includes bibliographical references (leaves 124-135) Dietary cysteine can replace up to 40% of the methionine requirement in the diets of both men and rats. Various experiments were designed to examine this phenomenon, including measurements of transsulfuration flux in isolated hepatocytes, hepatic enzyme and substrate analysis of sulfur amino acid metabolism, and cystathionine β-synthase (CBS) mRNA. In adult rats fed a high cystine diet for one week, transsulfuration flux was decreased by 40% as compared to animals fed a basal diet. This change in flux was mirrored by a similar change in CBS activity and mRNA. In young rats fed sulfur amino acid supplemented diets for one week, transsulfuration flux measured with 0.1 mM methionine fell by over 50% in cystine supplemented animals as compared to basal and methionine supplemented rats. Among the variety of results that followed, CBS activity decreased in rats supplemented with cystine compared to high methionine supplemented rats by 45%, while CBS mRNA decreased by almost 65% in these two groups. We also examined the effect that allosteric regulators S-adenosylmethionine and S-adenosylhomocysteine had on the enzymes of methionine metabolism. Our results indicated that SAM played a crucial role in regulating transsulfuration and remethylation of homocysteine via folate.
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