DEFICIENT TYPE-I PROTEIN-KINASE A ISOZYME ACT SYSTEMIC LUPUS-ERYTHEMATOSUS T-LYMPHOCYTES .2. ABNORMAL ISOZYME KINETICS

Systemic lupus erythematosus (SLE) T cells exhibit deficient type I pr otein kinase A (PKA-I) isozyme phosphotransferase activity, resulting in impaired phosphorylation of plasma membrane-associated proteins, To determine the mechanism of this isozyme deficiency, we studied 16 SLE subjects with a mean (+/- 1 SD) SLE disease activity index of 16.7 +/ - 8.8 anti 16 normal controls, Immunoblotting of type I regulatory (RI ) subunit protein in SLE and control T cells demonstrated no significa nt differences in the amount of protein, Analysis of isozyme kinetics in SLE T cells demonstrated a 2.2-fold increase in the Michaelis-Mente n constant, a 2.5-fold increase in the apparent association constant f or cAMP, a 3.8-fold decrease in the maximal velocity, and a reduction in the mean maximal binding of cAMP to the RI subunit compared with co ntrol T cells, Reduction of the Hill coefficient from 1.2 in normal T cells to 0.7 in SLE T cells indicated a loss of positive cooperativity between cAMP binding sites A and B. An increase in the apparent assoc iation constant for cAMP signifies relative resistance to cAMP, indica ting that higher intracellular concentrations of cAMP are necessary to activate the isozyme. Because the R subunit of PKA is the only intrac ellular receptor for cAMP, the abnormal isozyne kinetics may account f or the deficiency of PKA-I phosphotransferase activity and impaired PK A-I-catalyzed protein phosphorylation observed in SLE T cells, This di sordered isozyme function may contribute to the altered signal transdu ction and observed cellular immune dysfunctions in SLE, Moreover, thes e altered isozyme kinetics raise the possibility of a structural defec t(s) in the RE subunit.