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Glycogen Phosphorylase and Branching Enzyme Activity in Red and White Myofibers of Bovine Sternomandibularis Muscle

University of Guelph¹, Guelph, Ontario, Can.

Abstract

Glycogen phosphorylase activity was examined in myofibers from the bovine sternomandibularis muscle by microspectrophotometry of iodine-stained amylose synthesized from glucose-1-phosphate in sections of frozen muscle. Myofibers were categorized as either red or white, based on the strength of diphosphopyridine nucleotide tetrazolium reductase activity. Approximately equal numbers of red and white myofibers were found in the muscle. Immediately postmortem, red and white myofibers contained similar quantities of glycogen stainable by the periodic acid — Schiff reaction. Both red and white myofibers were able to synthesize amylose from a solution of glucose-1-phosphate buffered to pH 5.9. The synthesis of amylose in both red and white myofibers was greatly increased by activation with adenosine-5-monophosphate. The resulting amylose was stained brown in red myofibers (peak absorbance 460 to 540 nm, probably due to short chain length) and blue in white myofibers (peak absorbance 580 nm, probably due to long chain length). Inhibition of branching enzyme activity with ethanol reduced the brown coloration in red myofibers and increased the blue coloration in white myofibers. Both brown and blue amylose reaction products could be digested with β-amylase prior to staining and were considered to be derived from phosphorylase activity. These results suggest that, given adequate stores of glycogen immediately postmortem, red myofibers are capable of making a substantial contribution to postmortem glycolytic metabolism in dark or red beef muscles. Differences in glycogenolytic enzymes between red and white myofibers were suspected but their nature was not established.

¹ Department of Animal and Poultry Science.