Protein phosphatase inhibitor-1 was purified from bovine adipose tissue. The 
protein had an apparent molecular mass of 32 kDa by SDS/PAGE and a Stokes' 
radius of 3.4 nm. It was phosphorylated by cAMP-dependent protein kinase on a 
threonyl residue; this phosphorylation was necessary for inhibition of protein 
phosphatase-1. Bovine adipose tissue inhibitor-1 was compared directly with 
rabbit skeletal muscle inhibitor-1 and with a 32000-Mr, dopamine- and 
cAMP-regulated phosphoprotein from bovine brain (DARPP-32), also an inhibitor of 
protein phosphatase-1. By the following biochemical and immunochemical criteria, 
bovine adipose tissue inhibitor-1 was found to be very similar and possibly 
identical to DARPP-32 and was clearly distinct from skeletal muscle inhibitor-1: 
molecular mass by SDS/PAGE; Stokes' radii; phosphorylation on threonine 
residues; Staphylococcus-aureus-V8-protease-generated peptide patterns analyzed 
by SDS/PAGE; tryptic phosphopeptide maps analysed by two-dimensional thin-layer 
electrophoresis/chromatography; elution on reverse-phase HPLC; chymotryptic 
peptide maps as analysed by reverse-phase HPLC; amino acid composition; antibody 
recognition by immunoprecipitation and immunoblotting; effect of cyanogen 
bromide cleavage on protein phosphatase inhibitor activity. Based on these 
results we conclude that bovine brain and adipose tissue contain an identical 
phosphoprotein inhibitor of protein phosphatase-1 (DARPP-32), which is distinct 
from that of skeletal muscle (inhibitor-1).