Proglycogen and macroglycogen: artifacts of glycogen extraction?

Area of Science:

• Muscle physiology
• Biochemistry
• Glycogen metabolism

Background:

• Recent skeletal muscle physiology studies utilize homogenization-free acid extraction to differentiate proglycogen and macroglycogen.
• The suitability and accuracy of this protocol for assessing glycogen pools remain under investigation.

Purpose of the Study:

• To evaluate the appropriateness of the homogenization-free acid extraction protocol for muscle glycogen analysis.
• To determine if the acid-insoluble glycogen fraction represents proglycogen.
• To ascertain if the acid-insoluble fraction significantly contributes to changes in muscle glycogen content across different fiber types.

Main Methods:

• Optimized acid extraction protocol applied to rat skeletal muscle.
• Comparison of glycogen proportions obtained via optimized and homogenization-free methods.
• Gel chromatography analysis of acid-insoluble and acid-soluble glycogen fractions.

Main Results:

• Optimized acid extraction yielded 52%-64% acid-soluble glycogen, compared to ~16% with the homogenization-free method.
• Gel chromatography indicated no distinct difference between acid-insoluble and acid-soluble glycogen fractions, suggesting they are not separate pools like proglycogen.
• Acid-soluble glycogen levels fluctuated significantly during fasting-to-fed transitions, while acid-insoluble glycogen remained stable or slightly increased, irrespective of muscle fiber type.

Conclusions:

• The homogenization-free acid extraction method significantly underestimates the proportion of acid-soluble glycogen in skeletal muscle.
• Findings from studies using this protocol to investigate muscle glycogen physiology require critical re-examination due to potential inaccuracies.