Inherited abnormalities of red cell enzymes (red cell enzymopathies) are a distinct set of genetic disorders with one important clinical manifestation in common: chronic hemolytic anemia. Most of the enzymes involved are housekeeping enzymes present by definition in all cells. Therefore, one might expect that a severe reduction in activity of any of these might have generalized clinical manifestations. However, we can identify at least two reasons why red cells are more severely affected: firstly, red cells have a much more limited metabolic machinery than most other somatic cells; if a particular enzyme is deficient, other cells may cope by the use of alternate or surrogate metabolic pathways. Secondly, mature red cells are not competent for protein synthesis: therefore, if a particular enzyme is made highly unstable by a mutation, other cells can compensate by increased enzyme synthesis, but red cells cannot. Nevertheless, the fact that enzymopathies are not purely red cell disorders is highlighted by the coexistence, in some cases, of clinical manifestations in other systems, particularly the muscles and the nervous system; indeed, in some enzymopathies neurological damage may dominate the clinical picture.
This section deals with those enzymopathies affecting red cell metabolism for which the molecular basis has been elucidated. We will not discuss conditions in which an enzyme abnormality is expressed also in red cells but the main clinical manifestations are elsewhere (such as the porphyrias, galactosemia, and Lesch-Nyhan syndrome). For the sake of brevity, most of the enzymopathies will be discussed in groups.
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