Yolk degradation in tick eggs: I. Occurrence of a cathepsin L-like acid proteinase in yolk spheres.

In crude extracts of eggs of the soft tick Ornithodoros moubata, maximum degradation of vitellin is at pH 3-3.5, whereas no proteolysis is detected at neutral or weakly acidic pHs. Acidic proteolysis is maintained at high level throughout embryonic development, and rapidly decreases in the larva, during the high phase of yolk degradation. Proteinase, acid phosphatase, and N-acetylglucosaminidase are localized within the yolk spheres; these can be considered as lysosomal-like organelles containing both substrate (vitellin) and the degradative machinery. Proteolytic activity has been essentially attributed to a cathepsin L-like enzyme through substrate specificity and inhibitors. The molecular weight is 37,000 to 39,000 as shown using gelatin-containing SDS-PAGE activity gels. At neutral pH the enzyme binds to vitellin, as demonstrated by gel filtration and PAGE under nondenaturing conditions. Acid proteinase activity at pH 5-6 is undetectable both with proteins and synthetic substrates, but is strongly increased after preincubation at pH 3-4. Activation at low pH could be important in the regulation of yolk degradation.