Carbonic anhydrase isozyme distribution and characterization in metabolic fiber types of the dorsal levator muscle of the blue crab, Callinectes sapidus.

Abstract

Carbonic anhydrase (CA) distribution and characterization were examined in white and light pink fibers of the dorsal levator muscle of the blue crab. White fibers were structurally and metabolically characterized as fast twitch glycolytic, while the light pink fibers were fast oxidative. All subcellular fractions of both fiber types had significant levels of CA activity; cytoplasmic and microsomal activity was significantly higher in light pink vs white fibers. Cytoplasmic CA from both fiber types was highly sensitive to the inhibitors acetazolamide and chlorzolamide, with Ki values of approximately 2 and 0.4 nM, respectively. Further analysis confirmed that cytoplasmic CA from both fiber types was kinetically similar to the high turnover Type II isoform. It appears that the evolution of the CA Type III isoform, found in vertebrate red muscle, did not occur with the differentiation of metabolic fiber types in crustaceans. Membrane-associated CA, which was also kinetically similar to the Type II isoform, was 20-fold higher in light pink fibers, suggesting a physiological role in facilitated CO2 efflux from the muscle fiber during periods of prolonged maximal activity.