Effect of storage and in vitro ischemia on the ultrastructure of microsomal membranes and on microsomal enzymes

Abstract

Summary Pieces of liver (in vitro ischemia) and isolated microsomes were subjected to incubation at 4° C and 37° C for various time intervals. The effects on microsomal protein, phospholipids, and cholesterol and on microsomal phosphatases and electron transport enzymes were followed as a function of time and temperature. NADH-cytochromec reductase was very labile and was completely inactivated by 1 h, whereas G6Pase lost 50% of its activity after 2 h at 37° C. IDPase and NADPH-cyt. c red. were of intermediate susceptibility whereas cytochromes b₅ and P-450 were the most stable enzymes assayed. After 24 h of incubation of isolated microsomes at 37° C there was no significant detachment of membrane components (protein, PLP or cholesterol), indicating that the inactivation of the enzymes was not primarily attributable to their solubilization. Instead, experiments with¹⁴C-leucine and¹⁴C-glycerol prelabeled microsomes demonstrated that the proteins detached from microsomes during incubation originated mainly from the intravesicular space due to rupture of the microsomal membranes. The addition of a lysosomal extract during incubation did not alter either the rate of inactivation of the enzymes or the proportion of solubilized membrane components indicating that attack from the outside by proteolytic enzymes is not the mechanism for enzyme inactivation. There was no apparent correlation between the rates of inactivation of enzymes in vitro and their calculated half-lives in vivo or their postulated intramembranous localization. Ultrastructurally, enzyme inactivation was initially associated with alterations of the microsomal membranes, such as vesicle aggregation, membrane rupture, loss of unit membrane structure, and subsequently, thickening of membranes and transformation of the microsomes into nonrecognizable amorphous material.