ABSTRACT
Carbonic anhydrase IV is one of 12 active human isozymes and one of four expressed on the extracellular surfaces of certain endothelial and epithelial cells. It is unique in being attached to the plasma membrane by a glycosyl-phosphatiydyl-inositol (GPI) anchor rather than by a membrane-spanning domain. It is also uniquely resistant to high concentrations of sodium dodecyl sulfate (SDS), which allows purification from tissues by inhibitor affinity chromatography without contamination by other isozymes. This unique resistance to SDS and recovery following denaturation is explained by the two disulfide bonds. The 35-kDa human CA IV is a "high activity" isozyme in CO2 hydration activity, like CA II, and has higher activity than other isozymes in catalyzing the dehydration of HCO3 (-). Human CA IV is also unique in that it contains no oligosaccharide chains, where all other mammalian CA IVs are glycoproteins with one to several oligosaccharide side chains.Although CA IV has been shown to be active in mediating CO2 and HCO3 (-) transport in many important tissues like kidney and lung, and in isolated cells from brain and muscle, the gene for CA IV appears not to be essential. The CA IV knockout mouse produced by targeted mutagenesis, though slightly smaller and produced in lower than expected numbers, is viable and has no obvious mutant phenotype. Conversely, several dominant negative mutations in humans are associated with one form of reitinitis pigmentosa (RP-17), which we attribute to unfolded protein accumulation in the choreocapillaris, leading to apoptosis of cells in the overlying retina.
Subject(s)
Carbonic Anhydrase IV/biosynthesis , Carbonic Anhydrase IV/chemistry , Carbonic Anhydrase IV/isolation & purification , Structure-Activity Relationship , Animals , Carbonic Anhydrase IV/genetics , Carbonic Anhydrase IV/metabolism , Catalysis , Enzyme Stability , Gene Expression Regulation, Enzymologic , Humans , Isoenzymes/chemistry , Isoenzymes/genetics , Isoenzymes/metabolism , Mice , Mutation , Rats , Tissue DistributionABSTRACT
The localization of bovine carbonic anhydrase isozyme VI (CA-VI) was examined immunohistochemically in bovine mammary glands during early lactation period (after 2-3 days of postpartum) and dry period (at about 2 months preparturition in adults), and young calves (at 30 and 150 days after birth) using specific CA-VI antiserum. The immunoreaction for anti-CA-VI antiserum was very weak in the mammary glands in young (prepubescent) calves. In dry period, CA-VI was also weakly expressed in secretory epithelial (acinar) and ductal cells. In contrast, the reaction was intense in mammary gland cells in early lactation period. Dot blotting analysis indicated that anti-CA-VI reacted positively to beastings and mature saliva, but weakly or not at all to milk during the dry period or calf saliva, respectively. The intense expression of CA-VI in the mammary glands in early lactation period might compensate for low levels of secretion from functionally and structurally immature salivary glands in young calves.
