K Kumaran

Gluconeogenesis and Glucuronidation in Liver in Vivo and the Heterogeneity of Hepatocyte Function

In order to examine metabolic zonation in human liver, glycerol, which labels carbons 2 and 5 of glucose-6-P, and [1-C]lactate, which labels carbons 3 and 4 of glucose-6-P, in the process of gluconeogenesis, were infused intravenously into healthy subjects who ingested acetaminophen and had fasted 36 h. Distributions of C were determined in glucose in blood and in the glucuronic acid moiety of acetaminophen glucuronide excreted in urine. Ratios of C in carbons 2 and 5 to C in carbons 3 and 4 were significantly higher in blood glucose than in glucuronide. Since glucose and glucuronic acid are formed from glucose-6-P in liver without randomization of carbon, the differences in the ratios indicate that the pool of glucose-6-P in liver is not homogeneous. The glucuronide sampled glucose-6-P with more label from lactate than glycerol compared to the glucose-6-P sampled by the glucose. The apparent explanation is the greater decrease in glycerol compared with lactate concentration as blood streams from the periportal to the perivenous zones of the liver lobule. Glucuronidation is then expressed in humans relatively more in the perivenous than periportal zones and gluconeogenesis from glycerol more in the periportal than perivenous zones.

Estimates of Krebs cycle activity and contributions of gluconeogenesis to hepatic glucose production in fasting healthy subjects and IDDM patients

SummaryNormal subjects, fasted 60 h, and patients with insulin-dependent diabetes mellitus (IDDM), withdrawn from insulin and fasted overnight, were given phenylacetate orally and intravenously infused with [3-14C]lactate and 13C-bicarbonate. Rates of hepatic gluconeogenesis relative to Krebs cycle rates were estimated from the 14C distribution in glutamate from urinary phenylacetylglutamine. Assuming the 13C enrichment of breath CO2 was that of the CO2 fixed by pyruvate, the enrichment to be expected in blood glucose, if all hepatic glucose production had been by gluconeogenesis, was then estimated. That estimate was compared with the actual enrichment in blood glucose, yielding the fraction of glucose production due to gluconeogenesis. Relative rates were similar in the 60-h fasted healthy subjects and the diabetic patients. Conversion of oxaloacetate to phosphoenolpyruvate was two to eight times Krebs cycle flux and decarboxylation of pyruvate to acetyl-CoA, oxidized in the cycle, was less than one-30th the fixation by pyruvate of CO2. Thus, in estimating the contribution of a gluconeogenic substrate to glucose production by measuring the incorporation of label from the labelled substrate into glucose, dilution of label at the level of oxaloacetate is relatively small. Pyruvate cycling was as much as one-half the rate of conversion of pyruvate to oxaloacetate. Glucose and glutamate carbons were derived from oxaloacetate formed by similar pathways if not from a common pool. In the 60-h fasted subjects, over 80 % of glucose production was via gluconeogenesis. In the diabetic subjects the percentages averaged about 45 %.

On the lack of formation of L-(+)-3-hydroxybutyrate by liver.

Abstract The terminal carbon of palmitic acid, traced with 14C, is preferentially incorporated into carbon 4 of hydroxybutyrate formed by hepatocytes and perfused livers from 18- to 19-day-old rats and perfused livers from fasted adult rats. However, 14C from [13-14C]palmitic acid is incorporated into carbon 1 of the hydroxybutyrate to the same extent as any one of the first 12 carbons of palmitic acid as assessed with [1-14C]palmitic acid and [6-14C]palmitic acid. Therefore, the hydroxybutyrate is formed via hydroxymethylglutaryl-CoA, i.e., it is in the d configuration, and hydrolysis of l -hydroxybutyryl-CoA, the intermediate in the β oxidation of the palmitate, does not occur. Further, a negligible amount of 14C remains in hydroxybutyrate formed from 14C-labeled palmitic acid by isolated hepatocytes and perfused livers from the young rats, when the hydroxybutyrate is treated with d -(−)-3-hydroxybutyrate dehydrogenase to convert the d isomer to acetoacetate. Thus, l -(+)-3-hydroxybutyrate is not produced by rat liver as assessed using these preparations.

Ketone body production in diabetic ketosis by other than liver

To determine if ketone bodies, synthesized from fatty acids by tissues other than the liver, enter the circulation, rats in diabetic ketosis were injected with sodium [6,13-14C]palmitate. Hydroxybutyrate was isolated from the urine excreted by each rat and from an aqueous extract of its carcass. The distribution of 14C in the four carbons of hydroxybutyrate in the extract was the same as in the urine. The ratio of 14C in carbon 1 to carbon 3 of the hydroxybutyrate averaged 1.80 and averaged 1.31 in carbon 2 to carbon 4. Hydroxybutyrate when formed by perfused liver has the same carbon 1-to-carbon 3 ratio as carbon 2-to-carbon 4 ratio. The results indicate that hydroxybutyrate synthesized by tissues other than the liver mixes in the circulation with that synthesized by the liver and a portion of the mix is then excreted in the urine. The difference between the carbon 1-to-3 carbon ratio 3 and carbon 2-to-carbon 4 ratio calculates to an estimated minimum of 15% to 17% of the hydroxybutyrate in the circulation of the ketotic diabetic rat having tissues other than the liver as its source. Assuming the liver and kidneys are the sources of the ketone bodies in diabetic ketosis, the ketone bodies produced by the kidneys are not excreted into the urine without first entering the circulation.