CHARACTERISTICS AND SPECIFICITY OF THE INHIBITION OF LIVER GLUCOSE-6-PHOSPHATASE BY ARACHIDONIC-ACID - LESSER INHIBITABILITY OF THE ENZYME OF DIABETIC RATS

The effect of arachidonic acid (DELTA4Ach) on liver glucose-6-phosphatase (Glc6Pase) has been studied in vitro using untreated and detergent-treated microsomes prepared from fed and 48-h-fasted normal rats and from streptozotocin-induced diabetic rats. Glc6Pase of both untreated and detergent-treated microsomes (60 mug protein/ml) is inhibited by DELTA4Ach in a dose-dependent manner between 10-100 muM. The inhibition is very rapid and does not depend on preincubation of microsomes in the presence of DELTA4Ach. It does depend on the concentration of microsomal membranes and on the concentration of glucose 6-phosphate: it is more pronounced at low Glc6P concentrations than at high. As a consequence, the enzyme displays sigmoidal kinetics in the presence of DELTA4Ach. Hill coefficients (equal to 1 in the control experiments) of about 1.4 were determined in the presence of 50 muM DELTA4Ach, indicating a clear positive cooperative dependency of the Glc6Pase upon its substrate in the presence of DELTA4Ach. The DELTA4Ach inhibition is fully reversible in the presence of bovine serum albumin. The inhibition does not depend on the metabolism of DELTA4Ach through the prostaglandin synthase (cyclooxygenase) or arachidonate 12-lipoxygenase pathways since it is not affected by indomethacin and nordihydroguaiaretic acid. Several other unsaturated fatty acids are able to inhibit the enzyme within the same concentration range. In contrast, saturated fatty acids, the arachidonic acid methyl ester and numerous other lipid compounds containing esterified unsaturated fatty acids do not inhibit Glc6Pase within the same concentration range. The enzyme of fed rats was inhibited in the same manner as the enzyme of 48-h-fasted rats. However, Glc6Pase of untreated microsomes from diabetic rats was less inhibitable by DELTA4Ach than the Glc6Pase of normal rats. This difference does not persist after solubilization of the membrane lipids by detergent treatment.