CARBOHYDRATE AND LIPID METABOLIC PATHWAYS

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The long term objective is to develop and apply methods for quantitating pathways of carbohydrate and lipid metabolism. The specific aim is to develop a method for estimating rates of gluconeogenesis which is safe, reliable, definitive, easily performed, and broadly applicable, an apply it to the testing of hypotheses with regard to carbohydrate metabolism in physiological and pathological conditions with emphasis on the diabetic state. D2O is administered at a safe, well tolerated dose. In the fasted state hexamethylenetetramine, formed from 6 equivalents of formaldehyde, is used to measure the enrichment of the deuteriums bound to carbons 2,5 and 6 of glucose. Formaldehyde containing the deuterium bound to carbon 6 is formed on periodic acid oxidation of blood glucose. Formaldehyde containing deuterium bound to carbon 5 is formed on oxidation of xylose formed from the glucose. Formaldehyde containing deuterium at carbon 2 is formed by oxidation of ribitol-5-P formed from the glucose. In the fed state, the D2O is given with acetaminophen. Deuterium bound to carbon 2 and 5 of glucose formed from the glucuronic acid moiety of the excreted glucuronide is determined. The fraction of glucose output formed by gluconeogenesis in the fasted state and the fraction of glycogen formation by gluconeogenesis in the fed state, also to the ratio of the enrichment at carbon 5 to that in body water. The carbon 5 to carbon 6 ratio is a measure of the contribution of glycerol to gluconeogenesis and the extend of equilibration of the hydrogens of pyruvate with those in water. Absolute rates of gluconeogenesis in the fasted state are estimated by multiplying the fraction via gluconeogenesis by glucose output measured using tritiated glucose or (6,6-D2) glucose. In the fed state the quantity of glycogen formed is measured using MRI to estimate liver volume and 13CNMR glycogen concentration. In the fasted state MRI and NMR are used to relate estimates of rates of gluconeogenesis to rates of glycogenolysis. Measurements will be made in normal subjects, in subjects who are obese and in NIDDM and MODY subjects. The findings should bear on the role of the liver in producing hyperglycemia in the diabetic. This in turn bears on the design of therapeutic agents and the adequacy of insulin given systemically in returning metabolism, not just blood glucose concentration, to normal. The approach for the first time allows the giving of D2O in safe and convenient doses to obtain adequate labeling in glucose for measurements of the pathways of its formation and conversion to glycogen. Because of the small dose of deuterated water that has to be given and the small amount of glucose that is required for analysis, the approach should be useful in pediatric as well s adult clinical studies.