Glutamine synthetase isozymes in elasmobranch brain and liver tissues

Glutamine synthetase is present as isozymic forms in the elasmobranchs Squalus acanthias (dogfish shark) and Dasyatis sabina (stingray). Subcellular fractionation of elasmobranch brain and liver tissue shows the enzyme to be predominantly cytosolic in the former tissue and mitochondrial in the latter. For the cytosolic brain enzyme, the subunit Mr equals 42,000 in the stingray and 45,000 in the shark, as determined by sodium dodecyl sulfate-gel electrophoresis/Western blotting. The subunit Mr = 45,000 and 47,000, respectively, for stingray and dogfish mitochondrial liver enzymes. Translation of total brain RNA from both species gives immunoprecipitable nascent peptides of the same size as their respective mature enzymes. However, in liver tissue, translation of glutamine synthetase mRNA yields peptides of higher Mr than that of the mature enzymes. In dogfish liver, Mr = 50,000 for the translation product and, in stingray liver, Mr = 48,000. This suggests that the translocation of the enzyme into liver mitochondria may be via a signal or leader sequence mechanism. The larger liver isozyme of elasmobranch glutamine synthetase is found in kidney where it is also known to be mitochondrial. The smaller cytosolic isozyme occurs in retina, heart, gill, and rectal gland tissue as well as in brain.