The Composition and Metabolism of Microsomal and Mitochondrial Membrane Lipids in the Morris 7777 Hepatoma

Summary

Mitochondria and microsomes isolated from the Morris 7777 hepatoma demonstrated a markedly different phospholipid composition from the control organelle both with respect to the types present and the fatty acid composition. Hepatoma mitochondria contained lower amounts of cardiolipin than control liver (3.7% versus 5.2%). Some compensation in the amount of acidic phospholipids in the hepatoma mitochondria was made by an increase in phosphatidylserine (4.9% versus 1.3%). The major difference in microsomal phospholipids was an increase in the hepatoma sphingomyelin content. The level of polyunsaturated fatty acids in both hepatoma organelles was lowered, concomitant with an increase in the level of monoun-saturated fatty acid. Moreover, the usual distribution of saturated fatty acids at position 1 and polyunsaturated fatty acids at position 2 was not observed in hepatoma mitochondrial phospholipids. Force-area curves of the hepatoma phospholipids spread on a mono-molecular film demonstrated a smaller area per molecule than those from control liver mitochondria. The zeta potential of liposomes of the hepatoma mitochondria phospholipids (−45) was less than that of control mitochondria (−81) as determined by microelectrophoresis. In studies of phosphoglyceride metabolism, the calcium-stimulated phospholipase A activity of the hepatoma mitochondria appeared to be more readily expressed than the same activity in the liver organelle. The maximal activity was lower, however, than that in liver mitochondria. Hepatoma microsomes incorporated free fatty acids into monoacyl phospholipids, in the presence of an acyl-CoA generating system, to a lesser degree than liver microsomes. Additionally, frozen and thawed microsomes from the hepatoma 7777 had little capacity to incorporate free fatty acids, whereas, such treatment had no effect on liver microsomes. These results indicate that the fatty acid activating enzyme of hepatoma microsomes is more labile than that of liver. Both microsomal samples had phospholipase(s) which were active on membrane phospholipids, yet the hepatoma microsomes demonstrated no accumulation of the monoacylphos-phoglyceride product, unlike liver where one half of the product accumulated. This indicates that the precursor for reacylation in the Lands’ cycle is decreased in the hepatoma. The hepatoma stearoyl-CoA desaturate activity was elevated with respect to the liver microsomal enzyme. Moreover, the ⁶△desaturase was significantly decreased in hepatoma. We conclude from these results that the alterations in hepatoma phospholipid composition, both quantitatively and qualitatively, may be accounted for, in part, by the metabolic aberrations described here.

Supported by USPH Grants CA 14318 and CA 10729, and by the Forsyth Cancer Service.

Recipient of NIH Career Development Award AM 17392.