Ageing of Neurospora crassa. IV. Induction of senescence in wild type by dietary amino acid analogs and reversal by antioxidants and membrane stabilizers

Abstract

The extensional growth rate of wild-type 74 A8 N. crassa in the presence of various concentrations of 19 amino acid analogs was measured. Seven analogs were not inhibitory at concentrations in the range of one to 10mM. Of the remaining 12 analogs, nine inhibited growth in a novel way. The kinetics of growth in the presence of these analogs at 30° were characterized by seven sequential phases: (1) lag; (2) acceleration of growth rate; (3) steady-state growth rate; (4) exponential rate of decline of growthrate; (5) no growth or growth rate ⩽ 0.1 mm h −1; (6) acceleration of growth rate; and (7) steady state. At 33°, phases 6 and 7 did not occur and irreparable death of the clones occurred. The mechanism by which the clones acquired resistance at 30° appeared to involve a combination of physiological adaptation and cellular selection. Dietary application of either free radical scavengers or surface-active membrane ‘stabilizers’ alleviated or prevented the inhibition and deterioration of growth rate which occurred in the presence of the nine amino acid analogs. Culture with either 4-fluorophenylalanine or ethionine led to an increase of the activities of antioxygenic enzymes glutathione peroxidase, glutathione reductase and superoxide dismutase. The amino acid analogs that cause senescence and death of growing cells are known to be incorporated into proteins and such proteins are generally abnormal. Because a substantial fraction of cellular protein occurs in membranes and the proteins synthesized by mitochondria are exclusively intrinsic membrane proteins, we suggest that a primary consequence of errors in protein synthesis is the production of faulty membranes. The deterioration of such membranes with associated lipid autoxidation and free radical production proceeding as a chain reaction at an exponential rate may in itself contribute to the exponential rate of cellular deterioration which is characteristic of the ageing process. According to this hypothesis, dietary membrane stabilizers, free radical scavengers and antioxygenic enzymes protect cells from error catastrophy arising from the chain of events leading from membrane deterioration.