AGING OF NEUROSPORA-CRASSA .1. EVIDENCE FOR FREE-RADICAL THEORY OF AGING FROM STUDIES OF A NATURAL-DEATH MUTANT

A recessive mutant of N. crassa, called natural-death, was characterized by a decreasing clonal growth potential under all nutritional conditions and the irreversible cessation of growth. The primary molecular defect of this mutant was not known. Evidence presented here, based upon measurements of the activities and thermolabilities of several enzymes, suggested that faulty protein synthesis was probably not a cause of the senescence and death of the mutant. The relative times before the onset of senescence and death of mutant clones in the last 40% of their chronological life-span were prolonged 2 to 3-fold by either dietary antioxidants or selenite and the total life-span was increased by 40% to 80%. These compounds also alleviated the senescent morphology and enhanced biomass production. Senescing clones accumulated a green fluorescent pigment in situ, but dietary antioxidant nordihydroguaiaretic acid prevented this accumulation. The fluorescent pigment exhibited the spectral properties of lipofuscin, an end product of lipid autoxidation. Relative to wild type, mycelial extracts of the mutant exhibited a 2 to 4-fold excess of activities of the antioxygenic enzymes superoxide dismutase, glutathione peroxidase and glutathione reductase. Briefly reviewed are the roles of antioxygenic enzymes and antioxidants in their protection against cellular damage from lipid autoxidation and free radical reactions; and the major lines of evidence which appear to support a form of the free radical theory of ageing, encompassing the interrelated processes of membrane deterioration, lipid autoxidation and deleterious free radical reactions as the major causes of cellular deterioration.