Ageing of Neurospora crassa. IX. Microviscosity properties of mitochondrial membranes during normal and abnormal growth and development of an inositol auxotroph.

Microviscosity of mitochondrial membranes of an inositol auxotroph of Neurospora was measured by the method of Shinitzky, employing the fluorescent probe diphenyl-hexatriene. With high concentration of inositol, growth, morphogenesis, and cellular and biochemical phenotypes of the auxotroph are normal; whereas with low concentrations, these characteristics become abnormal and cellular and clonal senescence ensue. During normal growth and development, the critical temperatures of phase transition, the energies and volumes of fusion (delta E, V), and the microviscosities (n) at low and high temperatures changed in a cyclical "gaussian" manner; whereas the microviscosity at 25 degrees C remained constant. The normal developmental changes of the microviscosity properties were consistent with Brody's molecular packaging hypothesis, whereby the biochemical properties of conidia are pre-determined in conidiogenic hyphae. The microviscosity properties and their developmental change were closely correlated with other biochemical and biological properties such as the critical extremities of growth temperature, activity of membrane-bound cytochrome oxidase, and the stages of cellular differentiation. The thermodynamic properties of the membrane microviscosity support the genetic hypothesis that conidia and conidiogenic hyphae are more highly differentiated than growing hyphae. During abnormal growth and development, delta E and V of the liquid-crystalline phase underwent a precocious, but otherwise normal change at an early age; whereas subsequent cellular and mitochondrial senescence was accompanied by an abnormal increase of microviscosity and abnormally small delta E and V. With the results of other experiments and by analogy to proposed structural determinants of microviscosity properties of other biological membranes, a tentative interpretation of the molecular basis of the microviscosity properties and their normal and abnormal changes is derived. The effects of phospholipase treatment indicated that electrostatic interaction of phospholipid polar groups with membrane proteins may restrict mobility, increasing microviscosity and decreasing energies of fusion. Abnormal development or ageing of the membranes, leading to abnormally large n and small delta E, is probably a consequence of excessive lipid peroxidation and related abnormal changes of their structure.