ENZYME-ACTIVITIES AND MITOCHONDRIAL SUBSTRATE OXIDATION IN TOBACCO HORNWORM MIDGUT

1. Mitochondria were isolated from the three morphologically distinct sections of the tobacco hornworm midgut. Mitochondria derived from all sections oxidized pyruvate, organic acids and palmitoyl carnitine. Of the amino acids tested, only glutamate was oxidized. Generally, there was little difference among mitochondria isolated from different midgut sections. 2. Details of mitochondrial lipid oxidation were more carefully examined in mitochondria isolated from the posterior midgut section. Free fatty acids 3 to 16 carbons in length were oxidized in the presence of 0.5 mM malate. However, oxidation of fatty acids 14 to 18 carbons in length was increased by the presence of a carnitine ester. In contrast, fatty acyl carnitines 6 to 12 carbons in length were oxidized more slowly than the free fatty acids. Acetylcarnitine was oxidized more rapidly than acetate. 3. An enzyme profile of the tissue confirmed the mitochondrial findings. Enzymes involved in glycolysis or fatty acid oxidation were found in levels comparable to those of tissues which are known to oxidize lipids and carbohydrates. 4. Carbohydrate and lipid metabolism appears to be an important source of energy for the midgut tissue. The role of substrate oxidation in supporting in vitro active ion transport is discussed.