AJP - Cell Physiology, Vol 258, Issue 2 C344-C351, Copyright © 1990 by American Physiological Society
Glycogen phosphorylase in fish muscle: demonstration of three interconvertible forms
H. Schmidt and G. Wegener
Institut fur Zoologie, Johannes Gutenberg-Universitat Mainz, Federal Republic of Germany.
White skeletal muscle of crucian carp contains a single isoenzyme of
glycogen phosphorylase, which was purified approximately 300-fold to a
specific activity of approximately 13 mumol.min-1.mg protein-1 (assayed in
the direction of glycogen breakdown at 25 degrees C). Tissue extracts of
crucian muscle produced three distinct peaks of phosphorylase activity when
separated on DEAE-Sephacel. Peaks 1 and 3 were identified, in terms of
kinetic properties and by interconversion experiments, as phosphorylase b
and a, respectively. Peak 2 was shown to be a phospho-dephospho hybrid. The
three interconvertible forms of phosphorylase were purified and shown to be
dimeric molecules at 20 degrees C. At 5 degrees C, a and the hybrid tended
to form tetramers. The Mr of the subunit was estimated to be 96,400 from
sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hybrid is
kinetically homogeneous, and its kinetic properties are intermediate
between those of b and a forms. The b, hybrid, and a forms of phosphorylase
can be isolated from rapidly frozen muscle of crucian but in different
proportions, depending on whether fish were anesthetized or forced to
muscular activity for 20 s. Muscle of anesthetized crucian had 36, 36, and
28% of phosphorylase b, hybrid, and a forms, respectively, whereas the
corresponding values for exercised fish were 12, 37, and 51%. Results
suggest that three interconvertible forms of phosphorylase exist
simultaneously in crucian muscle and that hybrid phosphorylase is active in
contracting muscle in vivo.
