EVOLUTION OF ALANINE-GLYOXYLATE AMINOTRANSFERASE-1 PEROXISOMAL AND MITOCHONDRIAL TARGETING - A SURVEY OF ITS SUBCELLULAR-DISTRIBUTION IN THE LIVERS OF VARIOUS REPRESENTATIVES OF THE CLASSES MAMMALIA, AVES AND AMPHIBIA

As part of a wider study on the molecular evolution of alanine:glyoxyl ate aminotransferase 1 (AGT1) intracellular compartmentalization, we h ave determined the subcellular distribution of immunoreactive AGT1, us ing postembedding protein A-gold immunoelectron microscopy, in the liv ers of various members of the classes Mammalia, Aves, and Amphibia. As far as organellar distribution is concerned, three categories could b e distinguished. In members of the first category (type I), all, or ne arly all, of the immunoreactive AGT1 was concentrated within the perox isomes. In the second category (type II), AGT1 was found more evenly d istributed in both peroxisomes and mitochondria. In the third category (type III), AGT1 was localized mainly within the mitochondria with mu ch lower, but widely variable, amounts in the peroxisomes. Type I anim als include the human, two great apes (gorilla, orangutan), two Old Wo rld monkeys (anubis baboon, japanese maraque), a New World monkey (whi te faced Saki monkey), a lagomorph (European rabbit), a bat (Seba's sh ort tailed fruit bat), two caviomorph rodents (guinea pig, orange-rump ed agouti), and two Australian marsupials (koala, Bennett's wallaby). Type II animals include two New World monkeys (common marmoset, cotton -top tamarin), three prosimians (brown lemur, fat-tailed dwarf Lemur, pygmy slow loris), five rodents (a hybrid crested porcupine, Colombian ground squirrel, laboratory rat, laboratory mouse, golden hamster), a n American marsupial (grey short-tailed opossum), and a bird (raven). Type III animals include the large tree shrew, three insectivores (com mon Eurasian mole, European hedgehog, house shrew), four carnivores (d omestic cat, ocelot, domestic dog, polecat ferret), and an amphibian ( common frog). In addition to these categories, some animals (e.g. guin ea pig, common frog) possessed significant amounts of cytosolic AGT1. Whereas the subcellular distribution of AGT1 in some orders (e. g. Ins ectivora and Carnivora) did not appear to vary markedly between the di fferent members, in other orders (e.g. Primates, Rodentia and Marsupia lia) it fluctuated widely between the different species. Phylogenetic analysis indicates that the subcellular distribution of AGT1 has chang ed radically on numerous occasions during the evolution of mammals. Th e new observations presented in this paper are compatible with our pre vious demonstration of a relationship between AGT1 subcellular distrib ution and either present or putative ancestral dietary habit, and our previous suggestion that the molecular evolution of the AGT gene has b een markedly influenced by dietary selection pressure.