Insulin degradation by human skeletal muscle

Area of Science:

• Biochemistry
• Human Physiology
• Enzymology

Background:

• Previous research extensively characterized insulin degradation in animal tissues.
• Limited data existed on insulin-responsive human tissues, particularly skeletal muscle.

Purpose of the Study:

• To characterize the insulin-degrading activity in human skeletal muscle.
• To identify and purify the enzyme responsible for this activity.

Main Methods:

• Fractionation of skeletal muscle homogenate.
• Purification using ammonium sulfate fractionation, molecular exclusion, ion-exchange, and affinity chromatography.
• Enzymatic activity assays using 125I-labeled insulin and measurement of trichloroacetic acid-soluble products.

Main Results:

• A soluble enzyme, purified 1400-fold, showed high specificity for insulin degradation.
• The enzyme, a cysteine protease, had a Km of 1.63 x 10(-7)M and was inhibited by proinsulin and glucagon.
• Optimal activity was observed between pH 7-8, with a molecular weight of 120,000. A sulfhydryl group was essential for its function.

Conclusions:

• The major system for insulin degradation in human skeletal muscle is a soluble cysteine protease.
• This protease efficiently metabolizes insulin under physiological conditions.