Metabolic intermediate complexes properties

PA is a metabolic intermediate in the biosynthetic/degradation pathways of more complex glycerophospholipids. It usually represents less than 1% of total membrane lipids but plays a critical role in signal transduction, due to the xmique ionization properties of its phosphate group (see Chapter 3). Membrane glycerophospholipids are derived from PA by condensation with an organic alcohol (general formula X-OH). Phosphatidylcholine (PC), the most abundant membrane lipid, results from the condensation of choline with PA (Fig. 1.14). 

The vitamin B complex contains a number of factors which are closely associated in their distribution in nature and have related functions in intermediate metabolism. Of the eleven factors which are available in pure form, five have been shown to be constituents of coenzymes, namely, thiamine, riboflavin, niacinamide, pyridoxine, and pantothenic acid. It seems likely that other B vitamins may be found to function in a similar manner. Two members of the B complex, choline and inositol, appear to have lipotropic activity, and two others, folic acid and vitamin B12, have antianemic properties. Deficiency of vitamins of the B complex is one of the most frequently encountered syndromes of malnutrition in man. 

It is implicit in our discussion of compartmentation that we be able to accurately determine the concentration of intermediates within the various intracellular organelles, especially the mitochondria. Without this ability it is impossible to determine the flux rate of key compoimds across membranes. Is it possible, for example, to accurately determine the intramitochondrial concentration of oxaloacetate during periods of enhanced gluconeogenic flux in order to relate its levels to the known kinetic properties of the enzymes which compete for oxaloacetate Clearly, the formulation of any model for the control of a complex metabolic sequence must consider the actual concentration of all intermediates in the mitochondria if it is to achieve a meaningful interpretation of the kinetic properties of key enzymes. A simple measurement of the whole-cell concentration of an intermediate can be totally misleading, since intracellular gradients occur in various compartments. 

Metabolic breakdown of several benzodiazepine compounds commonly prescribed in the body leads to the production of several intermediate metabolites which have anxiolytic properties. Oxazepam occupies the position of a final common pathway in the metabolic breakdown process, further chemical changes producing inactive compounds. The failure of oxazepam to prevent diazepam withdrawal symptoms when it has been described as producing such symptoms on its own account (39 ) suggests another unresolved complexity in benzodiazepine dependence. 

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