The human red cell has a life span of 120 days. The mechanism that determines cell removal from the circulation with such precision remains unknown. Most studies of red cell aging have been based on analysis of cells of progressively increasing age separated by density. The relationship between red cell age and density has been recently challenged, and the hypothesis has been put forward that cell death is not the result of a progressive deterioration of essential cell constituents. This theory was based on preliminary observations in transient erythroblastopenia of childhood, which could not later be confirmed. When the relationship between cell aging and increasing density is critically reviewed, it appears to be based on firm experimental evidence, confirmed by in vivo demonstration of decreasing survival of cells of increasing age. Analysis of studies using buoyant density gradients reveals that this technique can easily distinguish the single exponential slope of decline for those cell components that change progressively throughout the red cell life span from the biphasic decline of those that decrease drastically at the reticulocyte-mature red cell transition. The view that the aging of the red cell and its removal from the circulation result from a progressive series of events during the 120 days of its life span appears to be the most consistent with the available data. Density separation, validated by much experimental evidence, remains a most useful technique for the study of the mechanism of aging of the red cell.

The existence of a five-membered isozyme system for human phosphofructokinase (PFK; ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) has been demonstrated. These multimolecular forms result from the random polymerization of two distinct subunits, M (muscle type) and L (liver type), to form all possible tetrameters-i.e., M4

Human erythrocytes were separated by buoyant density ultracentrifugation into fractions of progressively increasing mean cell age to measure the changes in glycolytic activity that occur during their 120-day life-span. The maximal activities of all glycolytic enzymes were shown to decline exponentially with cell age. Only three glycolytic enzymes exhibited a marked rate of decline with a tl/2 shorter than the cell life-span: hexokinase, aldolase, and pyruvate kinase. ganic phosphate), showed a fourfold decrease through the erythrocyte lifespan; lactate production also declined, but at a slower rate. When incubating conditions were altered by the introduction of a metabolic stimulus (either high phosphate for glycolysis, or methylene blue for the pentose pathway) the youngest cell fractions responded with decidedly increased rates of glucose consumption and lactate production. However, this ability gradually declined with cell aging, and ultimately, the oldest cells had metabolic rates as low as if there were no stimulus present, The oldest erythrocytes appear to have lost the flexibility needed to respond t o metabolic stress and are more vulnerable to events in the circulation that may require the ability to increase the basal rate. This defect is probably responsible for the disappearance of aged erythrocytes from the circulation.Glucose utilization, when measured in steady-state conditions (1 mM inor-