Reduced protein kinase C activity in sporadic Alzheimer's disease fibroblasts.

A decrease in the protein kinase C immunoreactivity and an altered protein phosphorylation have been reported in patients with Alzheimer's disease, but discordant results have been obtained from determinations of protein kinase C activity. By assaying the calcium- and phospholipid-dependent phosphorylation of a lysine-rich histone after detergent extraction, we have determined the total protein kinase C activity in fibroblasts from patients with sporadic Alzheimer's disease, age-matched controls and young subjects. The activity was not significantly different between young and aged controls, whereas it was significantly lower (0.70 +/- 0.12 vs 1.16 +/- 0.23 nmol/min/mg protein, P less than 0.01) in the patients. The total amount of protein kinase C estimated from the binding of phorbol dibutyrate to intact cells was also significantly lower (1.70 +/- 0.41 vs 2.48 +/- 0.54 pmol/mg protein, P less than 0.01). This decrease in protein kinase C activity suggests that abnormal protein phosphorylation might play a role in the pathogenesis of the disease.