[Characterization of endogenous phosphorylation substrates of sarcoplasmic reticulum fragments from fast skeletal muscles of the rabbit].

Separation of phosphorylated sarcoplasmic reticulum (SR) fragments by polyacrylamide gel disc electrophoresis in the presence of Na-DS revealed that the radioactivity is distributed in protein zones with molecular weights of 95,000 and 6000-8000. The phosphorylation of the protein with m. w. of 95,000 is Ca2+-dependent. The tryptic hydrolysis of the phosphorylated SR fragments from fast skeletal muscles results in a loss of radioactivity by 60-70%; phospholipase C from Clostridium welchii reduces the labelled phosphate content by 40-50%. The cAMP-dependent protein kinase inhibitor decreases the phosphorylation of both substrates. The substrate of phosphorylation with m. w. of 6000-8000 is not stained with Amidoschwartz 10B or Coumassie brilliant blue. Extraction by an acidified chlorophorm--methanol mixture results in a proteolipid with specific radioactivity exceeding that of the original preparation of phosphorylated SR membranes 3-4-fold. Thin-layer chromatography on Silufol plates and Silicagel KSK showed that the proteolipid is not chromatographically homogeneous after 2-fold precipitation by diethyl ether and is localized in a band with Rf varying from 0.6 to 0.8. The fluorescence spectrum of the proteolipid in a chlorophorm--methanol--HCl solution is represented by an assymmetrical structure-free band with a maximum at 350 nm. A possible role of phosphorylase b and proteolipid in manifestation of the functional activity of the SR fragments is discussed.