The pathway of urea synthesis of mammalian liver has proved to be an excellent model for the investigation of the intracellular organization of soluble enzymes. Studies of the behaviour and regulation of the enzymes of this pathway in situ (reviewed by Cohen et al., 1997) have conclusively shown that the latter operates as a tightly organized system, referred to by Srere (1987), as a metabolon. Intermediates are retained within the pathway and are channelled between sequential enzymes at every step, even across the mitochondrial membranes. By channelled we mean that the intermediates do not mix freely throughout the bulk aqueous phase of the cell.
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