Folic acid sources

In terms of amino acids bacterial protein is similar to fish protein. The yeast s protein is almost identical to soya protein fungal protein is lower than yeast protein. In addition, SCP is deficient in amino acids with a sulphur bridge, such as cystine, cysteine and methionine. SCP as a food may require supplements of cysteine and methionine whereas they have high levels of lysine vitamins and other amino acids. The vitamins of microorganisms are primarily of the B type. Vitamin B12 occurs mostly hi bacteria, whereas algae are usually rich in vitamin A. The most common vitamins in SCP are thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, choline, folic acid, inositol, biotin, B12 and P-aminobenzoic acid. Table 14.4 shows the essential amino acid analysis of SCP compared with several sources of protein. 

Spinach, salad, cereal germ, and bran as well as pulses are good sources of folic acid. Liver and yeast contain high amounts of this vitamin, too, but are not consumed frequently enough to be relevant for the coverage of daily requirements [1,2]. 

TABLE 63-2. Food Sources of Iron, Folic Acid, or Vitamin B125... 

Folding paperboard food cartons, 25 36 Folex, 13 43t, 52-53 Folic acid, 2 822 27 655 25 800-803 ascorbic acid and, 25 769 cofactor forms of, 25 801-802 dietary sources of, 25 803 drug-nutrient interactions involving, 25 802-803... 

Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body, but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing folate, more precisely DHF, from precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

A number of nitrogen heterocyclic, aromatic compounds, riboflavin 26, folic acid 27a and biopterin 27b, isolated from natural sources, are related in structure to natural redox enzyme cofactors. The electrochemistry of these and related compounds has been studied extensively. 

Cyanocobalamin, or vitamin B12, is in small amounts required for red blood cell production and for the formation of nucleoproteins and proteins. It is also needed for the proper functioning of the nervous system. Folic acid supplements can correct the anemia associated with vitamin B12 deflciency. Unfortunately, folic acid will not correct changes in the nervous system that result from vitamin B12 deficiency. Vitamin B12 is only found in animal sources such as liver and other organs. Some vitamin B12 is obtained from fish, eggs and milk. Folic acid and cyanocobalamin have been discussed in more detail in Chapter 22. 

B. Humans cannot synthesize folic acid (A) diet is their main source. Sulfonamides selectively inhibit microbially synthesized folic acid. Incorporation (B) of PABA into microbial folic acid is competitively inhibited by sulfonamides. The TMP-SMX combination is synergistic because it acts at different steps in microbial folic acid synthesis. All sulfonamides are bacteriostatic. Inhibition of the transpeptidation reaction (C) involved in the synthesis of the bacterial cell wall is the basic mechanism of action of (3-lac-tam antibiotics Changes in DNA gyrases (D) and active efflux transport system are mechanisms for resistance to quinolones. Structural changes (E) in dihydropteroate synthetase and overproduction of PABA are mechanisms of resistance to the sulfonamides. 

Mammalian cells (and some bacteria) lack the enzymes required for folate synthesis from PABA and depend on exogenous sources of folate therefore, they are not susceptible to sulfonamides. Sulfonamide resistance may occur as a result of mutations that (1) cause overproduction of PABA, (2) cause production of a folic acid-synthesizing enzyme that has low affinity for sulfonamides, or (3) impair permeability to the sulfonamide. Dihydropteroate synthase with low sulfonamide affinity is often encoded on a plasmid that is transmissible and can disseminate rapidly and widely. Sulfonamide-resistant dihydropteroate synthase mutants also can emerge under selective pressure. 

The active form of folic acid, tetrahydrofolic acid (THF), is produced from folate by dihydrofolate reductase in a two-step reaction requiring two moles of NADPH. The carbon unit carried by THF is bound to nitrogen N5 or N10, or to both N5 and N10. THF allows one-carbon compounds to be recognized and manipulated by biosynthetic enzymes. Figure 20.11 shows the structures of the various members of the THF family, and indicates the sources of the one-carbon units and the synthetic reactions in which the specific members participate. 

Vitamins and Minerals. Milk is a rich source of vitamins and other organic substances that stimulate microbial growth. Niacin, biotin, and pantothenic acid are required for growth by lactic streptococci (Reiter and Oram 1962). Thus the presence of an ample quantity of B-complex vitamins makes milk an excellent growth medium for these and other lactic acid bacteria. Milk is also a good source of orotic acid, a metabolic precursor of the pyrimidines required for nucleic acid synthesis. Fermentation can either increase or decrease the vitamin content of milk products (Deeth and Tamime 1981 Reddy et al. 1976). The folic acid and vitamin Bi2 content of cultured milk depends on the species and strain of culture used and the incubation conditions (Rao et al. 1984). When mixed cultures are used, excretion of B-complex vita-... 

Neither folic acid nor vitamin Bt is produced by humans in adequate amounts the substances must he absorbed from food. Natural sources of folic acid include ... 

Folic Acid. Citrus juice both in frozen or fresh form is a rich and stable source of folate. The presence of vitamin C in orange juice protects it from oxidation and, unlike other nutritional sources of folate, the folate in orange juice is not subjected to destruction caused by cooking of foodstuff. 

---