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THE OCCURRENCE AND OF MICROBODIES A Comparative Study STRUCTURE

Livers from chickens, rats, mongrel dogs, Dalmatian dogs, and man have been examined in the electron microscope in order to compare the microbodies with the known content of uricase. It is concluded that microbodies with inclusion are present in rats, mongrel dogs, and, although the inclusion generally is smaller, in Dalmatian dogs. The inclusion has a characteristic structural appearance. These species (rat, dog) have uricase. Chickens and man lack both the enzyme uricase and the microbody inclusion. This evidence and that from previously published electron micrographs in the literature on microbodies support the notion of a positive correlation between uricase and microbodies with an inclusion. It is recommended that the term "uricosome" be used for such microbodies that have an inclusion of the appearance here described. Microbodies are usually defined as cytoplasmic particles surrounded by a single membrane and having a size of 0.2 to 0.6 #. The term was introduced by Rhodin to denote certain particles in the tubule cells of the mouse kidney (1). According to Novikoff "the particles are best identified by the presence of an inner lamellated (crystalline?) body" (2). Such an inner body is not always present, however, and is absent from the material presented by Rhodin. Recently it has been shown that microbodies in the rat kidney have a crystalline inclusion, whereas those in mouse kidney do not (3). Microbodies with or without inclusions have been described from mammalian liver, hepatoma, and kidney. A suggestion as to their function comes from the work by de Duve and coworkers (4) who found that, during density centrifugation, uricase, catalase, and D-amino acid oxidase distribute themselves differently from both the lysosomal and the mitochondrial enzymes, and might thus belong to a separate class of cytoplasmic particles. The three enzymes do not become identically distributed, however; according to de Duve, this means that they "may either belong to separate particles showing very similar properties, or they are associated together in varying proportions with a single group of particles" (4). In subsequent electron microscope investigations, Novikoff (2) and Baudhuin and Beaufay (5) came to the conclusion that the particles containing "uricase and related activities" are the microbodies. This hypothesis may be tested by comparing livers from animals in which uricase is known to be present with those from animals in which uricase is absent with respect to the presence or absence of microbodies. The distribution of uricase is relatively well known among vertebrates and can be correlated, in turn, to both the phylogenetic relationships and the metabolic needs of the vertebrate classes. For a recent account of these interesting correlations the reader is referred to Baldwin's treatise (6). Uricase is known from fishes, amphibians, and