PURPOSE: To measure the titled isozyme simply in high accuracy, by using a protease having an activity to inhibit specifically aspartate aminotransferase isozyme.

CONSTITUTION: A protease having an activity to inhibit specifically aspartate aminotransferase (AST for short) isozyme is added to a test specimen such as serum, etc., cell-associated AST isoenzyme (s-AST for short) haivng different locality is deactivated, and only mitochondrion AST isozyme (m-AST for short) is measured. The difference between total AST and m-AST is obtained, so that m-AST is measure simply in high accuracy without separating two kinds of AST isozymes. Serine protease derived from a bacterium is preferable as the protease.