A case of M-type PFK deficiency due to the synthesis of a structurally abnormal and catalytically inactive M-subunit was reported. PFK activity was reduced (39% of normal) in red cells, normal in leukocytes and platelets, and absent in muscle. The red cell enzyme was not inhibited by antiserum to human muscle PFK and displayed normal biochemical properties (Km for ATP and fructose-6-phosphate, storage stability at +4°C and -80°C, optimum pH, electrophoretic pattern and molecular weight). The complete lack of PFK activity in muscle was confirmed on both histological preparations and muscle extracts. Double immunodiffusion analysis using an antinormal M-PFK serum revealed that the enzyme molecule was present and immunologically identical with normal, although it was catalytically inactive. The muscle abnormality was also confirmed by electromyography, ischemic exercise testing, histochemistry and electron microscopy. Moreover, PFK activity was investigated in myoblast cultures maintained up to 25 days, and it was found to be absent.
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