CARBONIC ANHYDRASE-III IN OBESE ZUCKER RATS

Proteins from 5- to 7-wk-old lean and obese Zucker rats were separated by one-dimensional sodium dodecyl sulfate (SDS) and two-dimensional S DS-isoelectric focusing-polyacrylamide gel electrophoresis. Laser dens itometry revealed an obesity-related decrease in the concentration of a 28-kDa cytosolic adipocyte protein, the most abundant protein in adi pocytes from lean Zucker rats. Microsequencing revealed the identity o f this protein to be carbonic anhydrase III (CA III). The identity and obesity-related decrease was further confirmed using isoform-specific antisera and CA III enzyme activity measurements made by O-18 mass sp ectrometry. Immunoblotting studies also revealed that CA III is presen t in at least two charge isoforms in adipocytes. Our data indicate tha t lean Zucker rat adipocytes may represent the richest source of CA II I in nature (24% of the cytosolic protein content). An obesity-related decrease in both the concentration and activity of CA III was observe d in two lipogenic tissues, liver and white fat, but not in soleus mus cle. Adipocyte CA III activity was no longer depressed when hyperinsul inemic obese rats were made insulin deficient by streptozotocin inject ion. This suggests that the obesity-related decrease in CA Ill may be related to the hyperinsulinemia as well as to the insulin hyperrespons iveness that adipocytes from obese Zucker rats of this age display.