Proteolysis of Japanese quail and chicken plasma apolipoprotein B and vitellogenin by cathepsin D: Similarity of the resulting protein fragments with egg yolk polypeptides

Plasma very-low density lipoprotein (VLDL) and vitellogenin (VTG) from mature female Japanese quail (Coturnix coturnix japonica) and chickens (Gallus domesticus) were isolated and digested in vitro with cathepsin D (EC3.4.23.5). The incubation mixtures were then reduced and subjected to gradient (4.5-18%) SDS-polyacrylamide gel electrophoresis. Protein fragments were stained with either Coomassie Brilliant Blue R-250 (VLDL digests) or Coomassie Brilliant Blue R-250 containing 20 mM AIC1₃ (VTG digests). Fragments resulting from the in vitro enzymatic digestion of quail and chicken plasma VLDL-apolipoprotein B (apo B) and VTG closely resembled those produced in vivo and isolated from egg yolks of each respective specie. Phosvitin, a proteolytically derived fragment of VTG, primarily existed as a single band (M_r ∼ 42 kDa) in Japanese quail yolk granules. In contrast, chicken phosvitin mainly consisted of a cluster of phosphoproteins ranging in size from -37 to 45 kDa. In addition to reporting a novel species difference in phosvitin moieties, the present study is the first to examine the role of cathepsin D in the generation of egg yolk proteins from plasma precursors in Japanese quail. Confirmatory evidence also was provided concerning the important role of this aspartic endopeptidase in the proteolytic cleavage of plasma VLDL-apo B and VTG in the chicken.