Adipose tissue thiamin

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

Storage in body Substantial primarily in liver, adipose tissue not found in all tissues Little or no storage (except vitamin B12 and possibly thiamin)... 

Figure 35.2 Effects of thiamine on adipocyte size in OLETF rats. Thiamine diminished adipocyte size, as shown by the distribution of this size in epididymal white adipose tissue obtained from the 55-week-old Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Open colunms, untreated control rats closed columns, thiamine-treated rats. Modified from Tanaka, Kohda et al. (2010).

Figure 35.2 shows the histograms of adipose cell size obtained from the untreated control and thiamine-treated 55-week-old rats. Histological evaluation revealed that smaller adipocytes in epididymal white adipose tissues were predominant in the thiamine-treated group compared with the untreated control group. 

Tissues which are more active in the synthesis of lipids than nucleotides require NADPH rather than ribose moieties. In such tissues, e.g. adipose tissue, the ribose 5-phosphate enters a series of sugar interconversion reactions which connect the pentose phosphate pathway with glycolysis and gluconeogenesis. These interconversion reactions constitute the non-oxidative phase of the pathway (Figure 11.14) and since oxidation is not involved, NADPH is not produced. Two enzymes catalyse the important reactions transketolase which contains thiamin diphosphate (Figure 12.3a) as its prosthetic group and transaldolase. Both enzymes function in the transfer of carbon units transketolase transfers two-carbon units and transaldolase transfers three-carbon units. The transfer always occurs from a ketose donor to an aldose acceptor. The interconversion sequence requires the oxidative phase to operate three times, i.e. three molecules of glucose 6-phosphate yield three molecules of ribulose 5-phosphate. 

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