Tricarboxylic acid cycle enzymes of the striped shore crab, Pachygrapsus crassipes

Abstract

1. 1. Spectrophotometric and manometric studies were undertaken to determine the specific activities, intracellular distribution and coenzyme specificities of some citric acid cycle enzymes in cheliped muscle and gill tissues of P. crassipes. 2. 2. The citrate/isocitrate ratios of mitochondrial aconitate hydratase activity were 71 for muscle and 96 for gill. The same ratios for the cytosol enzyme were 17 and 32. Three times more activity was found in the mitochondria of both tissues compared to the cytosol with citrate as substrate. However, there was an equal activity between these fractions with isocitrate as substrate. Muscle showed a higher overall enzyme activity with either substrate as compared to gill. 3. 3. The specific activities of mitochondrial and cytosol fumarate hydratases were similar in muscle and gill. The intracellular distribution of this enzyme expressed as the mitochondrial to cytosol (M/C) specific activity ratio was 3·0 for muscle and 3·4 for gill. 4. 4. No NAD-linked isocitrate dehydrogenase was detected in either cheliped muscle or gill. The M/C ratios of the NADP-specific enzyme were 0·79 and 0·65 for muscle and gill respectively. Total enzyme activity was higher in muscle. 5. 5. Succinate dehydrogenase M/C ratios for the oxidation of succinate were 9·5 for muscle and 8·4 for gill, whereas these ratios for the reverse reaction were 4·3 and 5·0 for muscle and gill respectively. The ratio of the mitochondrial enzyme activity in the oxidative vs. the reductive direction was 17 for muscle and 18 for gill. These ratios closely resemble those of aerobic organisms. Mitochondrial succinate dehydrogenase catalyzing the reduction of fumarate was four times more active with NADPH 2 than with NADPH 2 in both tissues. No activity of this enzyme with NADPH 2 was detected in the cytosol fractions. 6. 6. The results of this investigation indicate that the citric acid cycle in P. crassipes could be very similar to that of vertebrates with a few minor exceptions.