EVIDENCE FOR THE GLYOXYLATE CYCLE IN HUMAN LIVER

The enzymatic activities unique to the glyoxylate cycle of higher plants and certain lower invertebrates, isocitrate lyase and malate synthase, have been demonstrated in homogenates prepared from human liver. Human liver can also carry out cyanide-insensitive fatty acid oxidation from palmitate. Utilizing light microscopic immunocytochemistry with an antibody produced against Euglena malate synthase, this enzyme localizes in numerous ovoid granules in human hepatocytes. Also, immunocytochemistry using antibodies produced against rat fatty acyl-CoA oxidase showed that this enzyme was localized in similar structures. With routine cytochemistry, catalase was seen in identical granular bodies. Both catalase and fatty acyl-CoA oxidase are peroxisomal enzymes. The presence of malate synthase in liver homogenates was further confirmed by Western blot analysis. These data suggest that the human liver may be capable of utilizing the carbon backbone of fatty acids for carbohydrate synthesis since the glyoxylate cycle in lower organisms subserves this anabolic function.