Ov~idi (1991)
has written a concise and comprehensive review of the current literature which concerns the theoretical, teleological, and experimental aspects of channeling of metabolites in cells. The final answers concerning the existence and/or role of this metabolic event will come in the near future when technological developments will allow the engineering of cells to disrupt putative channeled paths and observation of metabolic effects (if any) in situ.
At the present time, Ovfidi points out that much heat without light is generated concerning the interpretation of in vitro experimentsunone of which are precisely comparable to each other let alone comparable to in situ cellular conditions.
In any short article it is easy to find "omissions" which are, not surprisingly, usually in the reviewer's field. In table 2 on evidence for channeling in the Krebs TCA cycle, I believe the work of Robinson et al. (1987) and Porpaczy et al. (1987) should have been cited, which shows higher rates of reactions of organized TCA cycle enzymes when compared to a soluble system of the same enzyme. In this same area of metabolism, channeling has recently been indicated in yeast cells by Siimegi et al. (1990) who showed that the metabolism of 3-~3C-propionate yielded alanine with unequal labeling in the C2 and C3 positions. Since the metabolic conversion is proposed to traverse part of the TCA cycle containing symmetrical intermediates one would have expected equal labeling in these carbons. Unless an unknown pathway for the conversion of propionate to alanine exists, one explanation for the unusual results would be that channeling occurs from succinyl CoA to oxalacetate.
It would have been interesting if the values for turnover times, transit times, and diffusion times between enzymes were compared. I believe they would indicate that response times for readjustment to changing metabolic conditions would appear quantitatively the most affected by the channeling process.
On the whole, I find the article of interest both to the novice and the expert in the area of metabolic channeling.