Metabolism and surface transport of parasitized erythrocytes in malaria

Plasmodium requires a living cell for growth and reproduction. Intraerythrocytically the parasite stores no reserve carbohydrate, relying entirely on host-supplied glucose and certain amino acids (glutamic acid) for its energy. Plasmodia are microaerophiles degrading glucose primarily to lactate rather than to CO2. The limited amounts of oxygen utilized may serve for biosynthetic purposes (e.g. pyrimidine biosynthesis) rather than being involved in an energy-yielding electron transport chain. Evidence for a parasite pentose pathway is weak since glucose-6-phosphate dehydrogenase has rarely been found; paradoxically, activity for 6-phosphogluconate dehydrogenase, the next enzyme in the pathway, is consistently identified. The parasites synthesize pyrimidines de novo, but being incapable of de novo purine biosynthesis they require preformed purines. Exogenously supplied purine, notably hypoxanthine derived from catabolism of erythrocytic ATP, is taken up and incorporated whereas pyrimidines are not. The capacity for de novo amino acid biosynthesis is limited and presumably haemoglobin supplies most of the amino acids required by the parasite. Degradation of haemoglobin, involving parasite proteases, notably a cathepsin D-like enzyme, leaves a characteristic golden-brown residue, haemozoin. Haemozoin consists of dimers of ferriprotoporphyrin IX, methaemoglobin and plasmodial proteins. For some species, isoleucine and methionine must be supplied exogenously for good plasmodial growth. Infected erythrocytes characteristically show altered permeability properties, changes which in large part contribute to parasite growth while at the same time impairing red cell function.