Abstract
Some properties of phosphodiesterases I (EC 3.1.4.1) from the plasma membrane of bovine mammary gland (PM) and from bovine milk fat globule membrane (MFGM) were studied and compared, using mostly p-nitrophenylthymidine 5'-phosphate (5'-PNPpT). The optimum pH was found to be 9.5-9.6 inethanolamine-HCl buffer. The enzymes from both PM and MFGM hydrolyzed 5'-PNPpT, p-nitrophenyl phenylphosphonate, and bis(p-nitrophenyl) phosphate but not p-nitrophenylthymidine 3'-phosphate. The Km of the enzymes was 1.7 × 10-3 M for PM and 1.6 × 10-3 M for MFGM using 5'-PNPpT. The activities of the enzymes from both membranes were inhibited by Mn, Zn, Co, Ni, and Cu ions and by EDTA, but unaffected by anions, and stimulated slightly by Mg and Ca ions. The properties of the enzyme of MFGM were very similar to those of the PM. The results suggest that the enzyme of MFGM is derived originally from that of PM, supporting the theory that MFGM is derived from the apical plasma membrane of the secretory cells.
