Substrate utilization in mammalian

Substrate Utilization in Mammalian Cells Lactate as Substrate... 

One is specific for benzoyl-CoA, sallcyl-CoA and short chain fatty acids while the other specifically utilizes phenylaoetyl-CoA or indol-3-ylacetyl-CoA (143). The substrates utilized by these mammalian enzymes are remarkably similar to the substrates metabolized to amino acid conjugates in plants. 

It will be noted that stigmasterol also represents a commercial route to progesterone. This abundant plant sterol that is readily available from soybean with its C22-C23 double bond, easily converts to progesterone in four steps and has therefore become a preferred substrate for various steroidal drugs. Cholesterol itself, which is the natural substrate for all mammalian steroid hormones, and is readily available from various sources (e.g., wool grease), is now also convertible to desirable steroid intermediates utilizing biotransforma-tion chemistry. The same is true of P-sitosterol (Chapter 11), which is another plant sterol available from soybean. 

While the carbon dioxide produced by decarboxylation of histidine can be measured by the standard Warburg manometer technique, the utility of the method is severely limited by the very small degree to which decarboxylation occurs with most mammalian histidine decarboxylase preparations even at high substrate concentrations. In practice, only bacterial histidine decarboxylases have proved sufficiently active to be measured conveniently by the manometric method . Small amounts of carbon dioxide can, however, be determined by the sensitive micro-diffusion technique of Conway , and this has been used successfully for measuring the activity of mammalian histidine decarboxylases . 

An excellent review of the energy sources utilized by mammalian embryos has been presented by Brinster (1971). His work has shown that the mouse embryo in vitro cannot be supported by glucose until the 8-cell stage of development. The preferred substrates for the culture of 8-cell stage embryos are pyruvate, lactate, and oxalacetate. 

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