A logical approach to the investigation of red cell enzymopathies.

This relatively rare group of disorders may cause quite marked morbidity and occasionally be life-threatening. As their inheritance is largely known accurate information in one family member has obvious benefits to other family members as well as the patient. The identification of the defect is dependent on an accurate clinical story which can be used to guide both the use and the interpretation of the various laboratory tests available. From the clinical aspect the enzymopathies can be divided into various broad groups. First, those involving the main glycolysis which produces the red cell's energy requirements in the form of ATP. Defects of this pathway generally cause a non-spherocytic haemolytic anaemia. Second, those involving the pentose phosphate shunt which maintains the redox potential of the cell necessary for its protection against oxidant stress. The commonest enzyme deficiency world wide, G6PD, is in this pathway and is characterized by stress-induced haemolytic crises. Third, defects of the various linked reactions. The most important of these are the methaemoglobin reductases which catalyse the reduction of methaemoglobin to functional haemoglobin and the enzymes in the Rapoport-Luebering shunt which can modulate the 2,3-DPG level. Whilst defects of these metabolic pathways make up the majority of cases associated with haemolysis, defects of other enzymes, on the whole less critical to the red cell's survival, must occasionally be considered. The red cell, because of its relatively easy availability, can be used as a 'biopsy tool' in the diagnosis of some systemic disorders in which the red cell enzymopathy is not the main feature of the disease. Such considerations are particularly important owing to the technological advances that have occurred in the last 10-15 years which have enabled correct assignment of an increased number of difficult cases. Not only is it possible to characterise the variant enzymes more accurately but it is now possible to have a 'metabolic window' on the red cell and examine it for the derangements of metabolism that characterise the various enzyme deficiencies.