Taurine is a dietary essential in the cat, which is an obligate carnivore with a limited capacity for taurine synthesis from cysteine. On a taurine-free diet, neither supplementary methionine nor cysteine will maintain normal plasma concentrations of taurine, because cats have an alternative pathway of cysteine metabolism: reaction with mevalonic acid to yield felinine (3-hydroxy-1,1-dimethylpropyl-cysteine), which is excreted in the urine. The activity of cysteine sulfinic acid decarboxylase in cat liver is very low.
It is not known to what extent taurine may be a dietary essential for human beings. There is little cysteine sulfinic acid decarboxylase activity in the human liver and, like the cat, loading doses of methionine and cysteine do not result in any significant increase in plasma taurine. This may be because cysteine sulfinic acid can also undergo transamination to ^-sulfhydryl pyruvate, which then loses sulfur dioxide nonenzymically to form pyruvate, thus regulating the amount of taurine that is formed from cysteine. There is no evidence of the development of any taurine deficiency disease under normal conditions.
There are very few plant sources of taurine, and strict vegetarians have a very low intake of preformed taurine. Nevertheless, the plasma concentration of taurine in strict vegetarians is generally between 40 to 50 ^mol per L, compared with concentrations between 55 to 70 pmol per L in omnivores.
In children undergoing long-term total parenteral nutrition without taurine supplements, there are changes in the electroretinogram similar to those seen in the taurine-deficient cat, suggesting that there is a requirement for some preformed taurine and that endogenous synthesis may be inadequate.
It has been suggested that preterm infants may require a dietary source of preformed taurine; breast milk initially contains a high concentration (about 300 pmol per L), and breast-fed infants maintain a higher plasma concentration of taurine than those fed on formula without added taurine (Chesney et al., 1998). Although milk from vegan mothers has a low concentration of taurine, and their infants have lower plasma concentrations and urinary excretion of taurine than the infants of omnivore mothers, there is no evidence that (full-term) infants of vegan mothers show any signs of taurine deficiency.
14.6 UBIQUINONE (COENZYME Q)
Ubiquinone functions as a carrier in the mitochondrial electron transport chain; it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1; Thomas etal., 1995, 1999).
There is no evidence that ubiquinone is a dietary essential, because it is synthesized in the body from mevalonate; indeed, dietary ubiquinone is relatively poorly absorbed (Dallner and Sindelar, 2000). However, because of its potential antioxidant action, supplements have been used, withlittle evidence
fully reduced quinol
ubiquinone semiquinone radical
Figure 14.8. Ubiquinone. Relative molecular mass (Mr): 863.3.
of efficacy, in the hope of preventing cancer and cardiovascular and neurodegenerative diseases (Beal, 1999; Hodges et al., 1999; Langsjoen and Langsjoen, 1999; Overvad et al., 1999; Thomas et al., 1999).
14.7 PHYTONUTRIENTS: POTENTIALLY PROTECTIVE COMPOUNDS IN PLANT FOODS
There is overwhelming epidemiological evidence that diets rich in fruit and vegetables are associated with a lower incidence of cancer, cardiovascular, and other degenerative diseases. To some extent, this may be because such diets provide less fat, and especially saturated fat, than diets that are richer in meat. The relatively high content of vitamins C and E and carotenoids in plant foods may also be important. In addition, fruits and vegetables contain a wide variety of compounds that have (potential) protective actions. These compounds are not strictly nutrients, in that they are not dietary essentials and have no physiological function.
Many fruits contain salicylates, which inhibit the synthesis of thromboxane A2, and have an anticoagulant action, in amounts that provide the same intake as the low dose of aspirin used as prophylaxis against thrombosis.
Plant sterols inhibit the intestinal absorption of cholesterol and so have a useful hypocholesterolemic action. They also inhibit endogenous synthesis of cholesterol, by inhibiting and repressing the regulatory enzyme of cholesterol synthesis, hydroxymethylglutaryl (HMG)-CoA reductase. Other compounds synthesized from mevalonate also inhibit and repress HMG-CoA reductase and have a hypocholesterolemic action, including squalene (found in relatively large amounts in olive oil), ubiquinone (Section 14.6), andthe tocotrienols (Section 4.1).
A number of the terpenes in aromatic oils of citrus peel, herbs, and spices inhibit the isoprenylation of the P21- ras oncogene product. Isoprenylation is essential for the biological action of the ras protein, which is associated with pancreatic cancer.
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Many women who have recently given birth are always interested in attempting to lose some of that extra weight that traditionally accompanies having a baby. What many of these women do not entirely realize is the fact that breast-feeding can not only help provide the baby with essential vitamins and nutrients, but can also help in the weight-loss process.