Effects of human serum on cathepsin D activity

The proteolytic activity of bovine splenic cathepsin D was evaluated by using the digestion of tritiated hemoglobin. Acid-denatured human serum was an enhancer of cathepsin D activity. Concentrated serum sources showed decreased activity because of competition of serum albumin with tritiated hemoglobin for cathepsin D. The enhancing activity of serum did not separate with any isolated protein as assessed by Sephadex G-50-80 chromatography or protein electrophoresis. The extraction of serum with ethanol-ether and chloroform-methanol showed that the enhancing activity separated with the phospholipid fraction, which had 2.6 times as much activity as the total lipid fraction. The activity was stable to heat at 56 degrees C for 30 minutes and to freeze-thawing, and was not dependent on metal ions.