Storage Pools and Turnover Systems in Growing and Non-growing Cells: Experiments with14C-Sucrose,14C-Glutamine, and14C-Asparagine

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Abstract
14C-labelled sucrose, glutamine, and asparagine have been supplied to aseptically cultured carrot explants that either grew rapidly by cell division or, by contrast, only slowly by cell expansion. The radioactive substrates were supplied in a brief ‘pulse’ followed by a much longer period during which the tissues were supplied with 12C-substrates. The passage of 14C through the various soluble compounds of the tissue and into the protein was followed. Alternatively, the 14C-labelled compound was supplied throughout the entire period of an experiment while the tissue also received 12C-sucrose. The pulse-labelling experiments demonstrate turnover and the fate of the breakdown products, as well as the emphasis placed on this kind of metabolism by cells at different levels of activity in their growth. The long-term labelling experiments show the different ways in which carbon from various sources may be used and how these pathways are affected by growth. The amount of 14C present in the various free (ethanol soluble) and combined (ethanol insoluble, acid-hydrolysable compounds—proteins) was determined, as well as the specific activity of the carbon in each compound. The fate of 14C supplied as sucrose had much in common with 14C supplied as glutamine, with respect to the ease with which it entered both the protein being synthesized and the carbon dioxide evolved, but it was very different from 14C supplied as asparagine. To interpret these data, compartments or pools of metabolites are postulated in the organized cell; exogenous 14C-sucrose and 14C-glutamine readily furnish carbon for pools of amino-acids en route to protein, which are protected from both the stored compounds and those which arise after protein breakdown. However, exogenous 14C-asparagine enters, is accumulated, and persists in the pool of stored compounds which also receive the nitrogen-rich substances that arise from protein breakdown. The kinetic data and the specific activities of the carbon in its various forms require that protein breakdown and re-synthesis occur concomitantly, that the stimulus to grow, exerted by coconut milk, accentuates protein synthesis and also the pace of its turnover, that some respired carbon dioxide arises from protein, and that this moiety of the respiration is increased by the coconut-milk stimulus as it accentuates the pace of cyclical turnover. In similar experiments with free cells from different plants, the same general conclusions apply, but the rates of turnover of protein are greater in free cells than in tissue explants. Some specific differences, however, exist. Cells of Arachis, the only legume investigated, permit 14C-asparagine to contribute, like 14C-glutamine, to both protein synthesis and respired 14CO2; it is not merely segregated in a storage pool. Thus, by virtue of their organization, plant cells maintain the same substances simultaneously in distinct phases or compartments, where they play distinctive roles, without mingling. Genetics endows each cell with the information that makes its biochemical reactions feasible; the organization of the cells determines how far the feasible becomes practised in cells in any given situation.