The Metabolism of Riboflavin

Photolysis of riboflavin occurs in vivo during phototherapy for neonatal hyperbilirubinemia (Section 7.4.4). There is no evidence that normal exposure to sunlight results in significant photolysis of riboflavin, although it is possible that some of the lumichromes found in urine may arise in this way. 

Lumiflavin and lumichrome edso catalyze oxidation of lipids (to Upid peroxides) md methionine (to methional), resulting in the development of an unpleasant flavor - the so-called sunlight flavor. 

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Bro-Rasmussen, F. (1958) The riboflavin requirement of animals and man and associated metabolic relations. II. Relation of requirement to the metabolism of protein and energy. Nutr. Abst. Rev. 369-86. 

Both drugs and compounds naturally present in foods may compete with vitamins for absorption. Chlorpromazine, tricyclic antidepressants, and some antimalarial dmgs inhibit the intestinal transport and metabolism of riboflavin (Section 7.4.4) carotenoids lacking vitamin A activity compete with /S-carotene for intestinal absorption and metabolism (Section 2.2.2.2) and alcohol inhibits the active transport of thiamin across the intestinal mucosa (Section 6.2). 

Riboflavin may also be involved in the metabolism of thyroid hormones. In the presence of oxygen, riboflavin phosphate catalyzes a photolytic deiod-ination of thyroxine. The lower tissue concentration of riboflavin phosphate in hypothyroidism may thus serve to protect such thyroid hormone as is available against catabolism and prolong its action. 

Creatinine excretion is intimately related to a relatively constant part of the body, the muscle mass. When studying the excretion of any metabolite, such as urea, calcium, or riboflavin, it might be undesirable to relate the amount of urinary metabolite to bodily weight, because the body contains compartments that are of minimal importance relative to metabolism. The body of an obese person contains a large amount of tissue (adipose tissue) that is inetaboiicaUy and biochemically irrelevant to the metabolism of compounds such as urea, calcium, and riboflavin. A person w hosc body contains excess body fluids also has extra mass that may be irrelevant to the metabolism of these as W cll as other compounds A meaningful comparison of excretion data from different subjects or from one subject at different times is facilitated by relating the data to urinary creatinine. 

Propazine and its primary metabolite, diamino-chlorotriazine, can attenuate the pituitary LH surge, leading to disruption of estrous cycle and certain reproductive and developmental processes. Propazine causes fatty degeneration. It also blocks metabolism of sugars and carbohydrates. It may also disturb the metabolism of some of the B vitamins (thiamine and riboflavin). 

Thiamine pyrophosphate has two important coenzyme roles, both of which focus mostly on carbohydrate metabolism (Figs. 8.26 and 8.27). The active portion of the coen- rae is the thiazole ring. The first step in the oxidative decarboxylation of a-keto acids requires TPP. The two most common examples are pyruvate and a-ketoglutarate, oxidatively decarboxyatedto acetyl CoA and succinyl CoA, respectively. The same reaction is found in the metabolism of the branched-chain amino acids valine, isoleucine, leucine, and methionine. In all cases, TPP is a coenzyme in a mitochondrial multienzyme complex, consisting of TPP, lipoic acid, coenzyme A, FAD, and NAD. Note the number of vitamins required for the oxidative decarboxylation of a-keto acids thiamine (TPP), pantothenic acid (coenzyme A), riboflavin (FAD),and niacin (NAD). 

Metabolic Role. Riboflavin coenzymes are required for most oxidations of carbon-carbon bonds (Fig. 8.29). Examples include the oxidation of succinyl CoA to fumarate in the Krebs cycle and introduction of a,jS-unsaturation in /3-oxidation of fatty acids. Riboflavin is also required for the metabolism of other vitamins, including the reduction of 5,10-methylene tetrahydrofolate to 5-methyl tetrahydrofolate (Fig. 8.49), and interconversion of pyridoxine-pyridoxal phos-phate-pyridoxamine (Fig. 8.33). Because oxi-dation/reductions that use FAD or FMN as the coenzyme constitute a two-step process, some flavin coenzyme systems contain more than one FAD or FMN. 

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