CYSTEINE PROTEINASES ARE RESPONSIBLE FOR CHARACTERISTIC TRANSKETOLASEALTERATIONS IN ALZHEIMER FIBROBLASTS

Cultured fibroblasts from patients affected by Alzheimer's disease (AD ) exhibited peculiar alterations of the enzyme transketolase (TK). Abn ormalities (dubbed alkaline line bands, ab) consisted of enzyme forms having unusually high pI and were proposed as a marker of the disease in living patients. The mechanisms of TK-ab expression were investigat ed with the use of cysteine proteinase inhibitors and purified prepara tions of either rat liver or human cysteine proteinases. The cysteine proteinase inhibitors N-acetyl-leu-leu-norleucinal (ALLN), L-trans-Epo xy-succinyl-leucylamido(4-guanidino)but (E-64), and egg white cystatin added to AD cells just prior to extraction abolished TK abnormalities . Moreover, 1 day incubation of AD cultures with either ALLN (10 mu g/ ml), NH4Cl (10 mM), or KCl (30 mM) prevented TK-ab generation, due, pr esumably, to an impairment of lysosomal functions. Isolated rat liver cysteine proteinases were able to degrade TK in normal extracts and re produce the characteristic TK-ab of AD fibroblasts. Moreover, pure hum an cathepsin H was also shown to partially induce an Alzheimer-like TK pattern and cleave normal TK to a 35 kDa fragment as spontaneously oc curring in AD fibroblasts. The explanation of mechanisms of TK-ab form ation provided evidence for an underlying imbalance of proteolysis in AD fibroblasts due to a relative increase/derangement of the cysteine proteinases cathepsins which might be also involved in the reported ab normal processing of multiple cellular components. (C) 1997 Wiley-Liss , Inc.