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.