Abstract
The insulin-like effects of ionic zinc (Zn2+) were studied in isolated rat adipocytes. Concentrations of Zn2+ between 250 and 1000 microM stimulated 3-O-methylglucose transport and glucose metabolism to CO2, glyceride-fatty acid, and glyceride-glycerol. Selective stimulation of the pentose phosphate cycle was observed since a Zn2+-induced increase in glucose carbon 1 oxidation persisted even when glucose transport was blocked with 50 microM cytochalasin B or when transport was no longer rate-limiting for metabolism at high concentrations of glucose. Enhanced pentose phosphate cycle activity may have been due to a selective inhibition of glutathione reductase by the ion, which was also accompanied by a fall in cellular glutathione content. Zn2+ also inhibited lipolysis stimulated by the beta-adrenergic agent ritodrine in the absence of glucose. The effects of Zn2+ on glucose oxidation and stimulated rates of lipolysis were inhibited by extracellular catalase, indicating that they were largely a result of H2O2 generation. The H2O2 production appeared for the most part to be caused by zinc-catalyzed autoxidation of sulfhydryl groups present on external cell membranes, although involvement of sulfhydryl groups on bovine serum albumin in the buffer could also have contributed. The insulin-like effects of Zn2+ in adipocytes are therefore caused not only by direct effects of the ion on intracellular metabolism but also by indirect effects related to H2O2 generation.
Highlights
The insulin-like effectsof ionic zinc(Zn2+)were stud- it seems unlikely that enhancement of insulinbindingacied in isolated rat adipocytes
In addition to the potential use of Zn2+as a tool with which to dissect the mechanism of insulin action, the insulin-like of Zn2+.The results indicate that Zn2+ mimicseveral actions of insulin including stimulation of 3-0-methylglucose transport, total glucose metabolism, and inhibition of stimulated lipolysis, and that a major portion of these effects are dueto H202generation
Stimulation of Glucose Metabolism by Zn2+-Adipocytes during both preliminary incubation and assay, while labeled 3-0- incubated with increasing amounts of Zn2+at low glucose methylglucose and L-glucose were added to cells only at the start of the assay
Summary
Complexed with Zn2+(2),and since insulin preparations used Materials-Bovine serum albumin (Cohn Fraction V, Lot S-11008). Displaced Cu3+or Fe3+could function as catalysts for sulmydryl Assay of Glucose Metabolism and Pentose Phosphate Cycle Acautoxidation, cause the H202 generation, and account for the tiuity-Glucose metabolism to C02, glyceride-fatty acids, and glycermajor observations of the present work Such a role for trace metal ide-glycerol was determined as follows. Hz02 production was measured in buffer with a to decrease scopoletin fluorescence somewhat, as shown by results of fluorometric horseradish peroxidase-scopoletii assay [30].The lower treatment in the absence of albumin. This effect was small relative limit of Hz02 detection was 0.05-0.08 m o l of H202/ml. Measurement of Hexose Transport-The assay of 3-0-methylglucose transport was performed under equilibrium exchange conditions, in which unlabeled 3-0-methylglucose and L-glucosewere present
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