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In eukaryotes, multienzyme complexes containing five to nine aminoacyl-tRNA synthetase activities have frequently been reported. In this study, we report the existence, in bovine liver cytoplasm, of a multienzyme complex containing at least 16 activities which can be disrupted by homogenization to give rise to smaller complexes and noncomplexed synthetases. Determination of the size and component activity of these complexes and of the molecular weights of all 20 free synthetases suggests that the smaller complexes and free activities normally identified arise from the larger complex by well-defined stages during homogenization. We also show that similar, though not identical, complexes are found in bovine liver mitochondria and give the molecular weights of 16 mitochondrial synthetases.

Summary. The extensive association of mammalian insulins in solution and the aggregation of insulin receptors in cell membranes are well documented. The hypothesis advanced here is that a direct connection exists between these observations. It is postulated that, after binding to its receptor, an insulin monomer can interact with another similarly bonded hormone-receptor complex through those groups on the insulin monomer faces utilized for dimer-dimer contacts in the crystal and in solution. Regarded thus, the insulin molecules are effectively bivalent as required for the formation of cross-links between receptors, with the accompanying enhancement of biological activity. A number of properties of native insulins from different animals, and of modified insulins, are considered in the light of this suggestion. It is shown to have considerable power in reconciling a diversity of such observations and to provide a plausible model for the experimentally observed receptor clustering phenomenon.Key words: Insulin, insulin activity, insulin association, crosslinking, receptor clustering.The primary event in the biological activity of insulin is the binding of an insulin monomer to a specific cell membrane receptor site [1]. There is convincing evidence, both from experiments involving visualization of fluorescently-labelled insulin derivatives [2], and from those utilizing other multivalent molecules giving rise to insulin-like effects [3], that the initial binding is followed by the formation of clusters of insulin-receptor complexes. Next, the hormone is apparently internalized by endocytosis [1,4] and, eventually, it is degraded [5]. It seems likely that the clustering accompanying the formation of the insulin-receptor complexes leads to local disruption of the membrane structure [1, 2] and hence is an important factor in the biological activity of insulin in promoting the transport of small molecules, particularly glucose, into the interior of the cell. It has been suggested [1, 2, 5] that some of insulin's other effects, for example, on protein biosynthesis and down-regulation of receptors, may depend on its internalization.De Lisi [6] has recently shown theoretically that ligand-induced receptor clustering is capable of producing very large signal amplification in the transduction of information at the cell surface. The situation discussed in his work is directly relevant to that outlined above for the insulin-receptor interaction and the magnitude of the effects which can be generated indicates the need for understanding the details of the process in specific systems. In the present paper I shall propose a model which displays the potential of insulin, in terms of its known molecular properties, for cross-linking receptors into an extensive network following the initial binding of insulin monomer to receptor. The proposed mechanism is shown to account for a wide variety of experimental observations of the relationship between insulin structure and activity, both with respect to comparisons of insulins from dif...

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