1. The erythrocytes of seven patients conforming to the criteria of Type II congenital nonspherocytic hemolytic anemia have been demonstrated to have a specific deficiency in the glycolytic enzyme pyruvate kinase. Other glycolytic enzymes, glucose-6-phosphate and 6-phosphogluconic dehydrogenases, and certain non-glycolytic erythrocyte enzymes are normally active. The leukocytes of these patients possess normal pyruvate kinase activity.

2. Although no inhibitors were detected, the exact nature of the enzymatic defect remains to be elucidated.

3. Family studies provide strong evidence for a genetically determined disorder and are consistent with an autosomal recessive transmission of the defect. A partial enzyme deficiency, not reflected in clinical disease, is present in heterozygotes. The symptomatic disease, though variable in severity, appears to be due to homozygosity for the defect.

4. It is suggested that the enzyme deficiency is pathogenetically related to the premature demise of the red cells in vivo.

5. The name "pyruvate kinase (PK) deficiency hereditary nonspherocytic hemolytic anemia" is proposed for these patients.

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