Liver Mitochondrial Carbonic Anhydrase (CA V), Gluconeogenesis, and Ureagenesis in the Hepatocyte

Abstract

In the 1970s, there was great interest in the functioning of isolated mitochondria, which led in 1978 to the awarding of the Nobel Prize for Physiology and Medicine to Dr. Peter Mitchell for his chemiosmotic theory. A few weeks later, the reviewer arrived in Philadelphia as the postdoctoral fellow of Dr. Robert E. Forster II. Dr. Forster has had a successful career devoted to studying anomalies in the theories of gas diffusion, pH disequilibrium, and carbonic anhydrase (CA)18,22; he believed that there was something wrong in Dr. Mitchell’s theory and that a mitochondrial CA would dissipate the proton gradient required. A collaboration was started with two experts in mitochondrial bioenergetics: Dr. Leena Mela, who knew how to isolate mitochondria from several organs, and Dr. Bayard Storey, who knew how to isolate skeletal muscle mitochondria.42 By 18O mass spectrometric CA analysis,18,30 the reviewer found abundant CA activity in guinea pig liver and skeletal muscle but none in brain, kidney, or heart.15 The work on skeletal muscle that has been done since then is reviewed elsewhere,42 as is the work with the CA V containing rat kidney mitochondria.6 Exhaustive literature searches indicated that liver fractionation by previous workers had given some evidence of CA activity in mitochondria, but several believed that this was the result of contamination from the cytosol of hepatocytes or erythrocytes (reviewed in reference 21). Work in which sulfonamide inhibition reduced HCO3−-linked Ca2+ transport across the mitochondrial membrane did, however, convince the investigators of the existence of a mitochondrial CA.17,24 With one notable exception,45 there has been until recently little interest outside of this laboratory in studying this lovely isozyme directly but considerable interest in studying the effects of rendering it nonfunctional by sulfonamide CA inhibition. Mitochondrial CA inhibition has been concluded to be responsible for decreased urea and glucose synthesis by alligators and chameleons in vivo,5 decreased urea synthesis by isolated perfused rat livers,25,34 decreased urea and glucose synthesis by isolated rat hepatocytes,3,35,39 and by isolated guinea pig hepatocytes,7,10,11,14 and decreased citrulline synthesis by intact isolated liver mitochondria from guinea pigs7,12 and rats46 (S. J. Dodgson and A. J. Meijer, unpublished results). Reviews of ongoing work in the field have appeared in the past decade.14,21,22