Carrier-mediated urea transport across the mitochondrial membrane of an elasmobranch (Raja erinaced) and a teleost (Oncorhynchus mykiss) fish

摘要

In piscine species, arginase is the key O-UC enzyme responsible for the production of urea from arginine. Although it is a ubiquitous enzyme, the mitochondria of the liver and kidney has the highest level of activity (4, 18, 23, 27, 43, 51, 58, 87). In contrast, in mammals, the urea precursor, arginine, is shuttled out of the mitochondria to interact with a cytosolic form of arginase (reviewed in Ref. 3). The fundamental differences in subcellular arginase localization suggest that each vertebrate class may require different mechanisms to regulate the movement of urea across the mitochondrial membrane. In fish, urea-produced arginase would be shuttled out of the mitochondria, whereas, in mammals, urea is produced in the cytosol. Although the outer mitochondrial membrane is freely permeable to molecules up to 1.5 kDa because of the presence of large pores formed by the voltage-dependent anion channel (VDAC; 21), it is the inner membrane that may pose a challenge in fish, since this boundary acts as a major barrier to solutes and water moving between the cytoplasm and mitochondrial matrix.