Concentration of liver and kidney fructose-1,6-diphosphatase determined by specific radioimmunoassay.

A radioimmunoassay for liver fructose-1,6-diphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrlase, EC 3.1.3.11) has been developed based on maintenance of its tetrameric structure and immunologic integrity after iodination by the Bolton-Hunter technique. The assay detected as little as 2 ng of standard enzyme. Nonspecific interference by tissue components did not occur. Enzyme concentration (mumol/1000 g tissue wet weight) was measured in tissue extracts of 49 rabbits subjected to a variety of conditions. In animals fed a 'balanced' diet containing 50--60% carbohydrate (by weight), the concentration in liver was 3.4 microM +/- 0.3. After fasts of 48, 72, or 96 h, the concentration in liver increased approximately 1.4-fold. A high-fat diet did not alter the concentration significantly but a high-protein diet caused an increase of 2.1-fold to 7.2 microM +/- 1.4. The greatest concentrations, 8.7 microM +/- 1.9, were observed in the livers of severely diabetic rabbits. The increase paralleled the increasing severity of diabetes and provides one explanation for the augmented gluconeogenesis which occurs in the diabetic state. Changes were less marked in kidney. The greatest apparent incrase, from 2.6 microM +/- 1.1 in the normal fed rabbit to 4.7 microM +/- 2.8, occurred in the severely diabetic animal. However, variation was sufficiently great in kidney to render apparent increases during fasting, protein feefing and diabetes statistically insignificant. For the most part changes in assayable activity followed changes in enzyme concentration except in the rabbits maintained on high-protein diets. In these, liver enzyme concentration increased by 2.4-fold whereas activity increased by only 1.3-fold, and the kidney enzyme concentration increased 1.3-fold whereas activity decreased by 20%.