Lactic acid metabolic groups

Several carrier systems have been shown to be present in the brain endothelium, allowing for the selective transport of a group of common substrates (Table 13.1). The most common system is the one that mediates the transport of glucose, which provides the brain with virtually all its energy. Carrier-mediated mechanisms are also responsible for the absorption of two other energy sources ketone bodies, which are derived from lipids, and lactic acid, a by-product of sugar metabolism. Carrier-mediated transport systems are also involved in the uptake of amino acids by the brain. The brain can manufacture its own small neutral and acidic amino acids however, large neutral and basic amino acids are obtained from the bloodstream. 

The human metabolism makes use of the oxidation of alcohols to render harmless other toxic compounds containing the OH group. For example, lactic acid, produced in muscles during intense activity, is oxidized by an enzyme called lactate dehydrogenase to the metabolicaliy usefui compound pyruvic acid. 

Nicotinamide adenine dinucleotide (NADH) supplies a hydride ion for reductions of carbonyl groups in biological systems. NADH is converted to NAD" in the process. An example is the reduction of pyruvic acid to lactic acid, which occurs during the metabolism of glucose ... 

C20H23N7O7, Mr 473.45, pale yellow crystals +3 H2O, mp. 248 -250 °C (decomp.), [a]n +16.8° (aqueous hydrogen carbonate). F. occurs in various microorganisms in which it acts as a growth factor e. g., for the lactic acid bacterium Leuconostoc citrovorum, therefore, it is also known as citrovorum factor. F. functions as an active 1-C unit (cf. folic acid) and formyl group transfer agent in the metabolism of folic acid tetrahy-drofolic acid. F. is accessible by synthesis and its calcium salt is used therapeutically as an antidote for folic acid antagonists such as methotrexate. 

Table 13.1 Lactic acid bacteria metabolic groups and tbeir characteristics (Hammes Vogel, 1995)...

Lactic acid bacteria have very limited synthetic capabilities and, reflecting this, the group is frequendy described as being fastidious. Simply stated, this means that LAB have complex nutritional requirements that they are metabolically incapable of meeting. This is seen in their habitat specificity which is restricted to decomposing plant tissue and in association with other microbes (i.e., yeasts) where they are able to glean decomposition products for their own needs. 

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