Vitamins, continued accumulation

Biosynthesis of vitamin Bn by propionic acid bacteria, as we have seen (Section 4.4.5), almost parallels growth under anaerobic conditions. Vitamin Bn accumulates in the cell (the above-mentioned case of vitamin Bn secretion apparently is peculiar to a single mutant strain) mainly as coenzyme forms of incomplete corrinoids. These features are taken into account in the industrial production of vitamin Bn. The culture is grown anaerobically, in a medium containing glucose, corn-steep liquor, ammonium sulfate, and a cobaltous salt, since Co is part of the vitamin Bn molecule. Acids formed by the culture are continuously neutralized with a solution of NaOH or (NH4)20H. 

At One time it was thought that women taking oral contraceptives were at risk for B deficiency. This notion seem-S to have been in error. The error was due to a misinterpretation of the tryptophan load lest. As mentioned earlier, a deficiency in vitamin B(,can induce the accumulation of specific intermediates of the tryptophan catabolic pathway and enhanced excretion in the urine. Oral contraceptives can also induce ar increase in the formation and excretion of specific intermediates by stimulating the activity of specific enzymes of the tryptophan catabolic pathway, This stimulation was responsible for the false indications of deficiency. Independently of the tryptophan load test, there continues to be some evidence for risk associated with the use of oral contraceptives. Oral contraceptive use may result in lowered levels of plasma vitamin Bf, Tlicsc lowered levels may result in a vitamin deficiency when coupled with pregnancy and lactation. 

Glutathione is discussed further in the section on selenium and glutathione in Chapter 10. The enzyme assay is conducted using glutathione reductase extracted from red blood cells with and without added FAD. Chmnic consumption of a diet deficient in riboflavin allows the continued synthesis of a variety of flavoproteins, but results in the accumulation of apoenzyme without its conversion to holoen-zyme. Addition of chemically pure FAD to a biological fluid containing apoenzyme results In the stimulation of enzyme activity because of the formation of the holoenzyme. It is this stimulation of enzyme activity that is used to determine vitamin status in humans. 

Tissue accumulation of a-tocopherol in vitamin E supplemented animals appears to occur in a dose-dependent manner. Since a-tocopherol is consumed in performing its antioxidant role, the strategy for gaining maximum shelf-life is to maximize the concentration of vitamin E present in tissue at slaughter. Arnold et al. (1992b) have established that beef muscle can become saturated with a-tocopherol and that continued supplementation of animals with vitamin E beyond this point yields no added benefit and results in unnecessary expense. 

Several vitamins are not stored. Several of the vitamins cannot be accumulated and stored in the body for any considerable period of time they are used up quickly and therefore must be constantly replaced or renewed with a continuing supply from the outside. 

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