Enzymatic characterization of cathepsin D in rabbit brains with experimental neurofibrillary changes.

Aluminium salt was injected intracerebrally into rabbits resulting in experimental neurofibrillary change (ENFC) formation as a model of Alzheimer neurofibrillary change. Cathepsin D was purified from the experimental and the control rabbit brains and the enzymatic properties were further investigated. The specific activity of cathepsin D in the tissue homogenate from the ENFC brains was 27% higher than that of the control, reflecting the induction of the enzyme by aluminium injection. The apparent Km values of the control and ENFC enzymes were calculated to be 26.3 microM, 29.3 microM, respectively, when assayed with bovine hemoglobin as substrate. The heat inactivation proceeded linearly with time for the control enzyme but biphasically for the ENFC enzyme, suggesting that less-reactive type of cathepsin D was induced by aluminium injection. Other properties such as molecular weight, optimum pH and amino acid composition were similar to each other.