Dturing the early period of the twentieth century, it was generally thought that alkaloids were byp)roducts of a number of irreversible and physiologically useless reactions associated with nitrogen metabolism (1-3). This idea was gradually discarded as an increasing number of experiments showed that alkaloids are metabolically active.Indirect evidence for the decomposition of ricinine (II, fig 1) (7) has been made on the degradation of ricinine in a cell-free system of young castor plants. Tso and Jeffrey (8) were the first to use isotopic tracers to demonstrate that alkaloids are metabolically active. They fed nitrogen-15 labeled nicotine (III, fig 1), nornicotine and anabasine (IV, fig 1)

Summary Understanding the mechanisms for cellular aging is a fundamental question in biology. Normal red blood cells (RBCs) survive for approximately 100 days, and their survival is likely limited by functional decline secondary to cumulative damage to cell constituents, which may be reflected in altered metabolic capabilities. To investigate metabolic changes during in vivo RBC aging, labeled cell populations were purified at intervals and assessed for abundance of metabolic intermediates using mass spectrometry. A total of 167 metabolites were profiled and quantified from cell populations of defined ages. Older RBCs maintained ATP and redox charge states at the cost of altered activity of enzymatic pathways. Time-dependent changes were identified in metabolites related to maintenance of the redox state and membrane structure. These findings illuminate the differential metabolic pathway usage associated with normal cellular aging and identify potential biomarkers to determine average RBC age and rates of RBC turnover from a single blood sample.