The formation of new skin cells is accompanied by the synthesis of DNA; hence, these are the dynamics of quantifying the rate of DNA turnover. DNA (e.g., adenosine) can be synthesized from the de novo synthesis starting from the formation of the nucleotide base; the de novo synthesis of deoxyribose and its reaction with intact base; and the reuse of intact deoxyribonucleotides, the so-called deoxyribonucleoside salvage pathway. These pathways are graphically shown in Figure 4.1. Traditionally, DNA synthesis had been measured in vitro using either 3H-thymidine or bormode-oxyuridine as the tracer. These tracers are toxic and are not suitable for application to the measurement of the in vivo DNA synthesis rate. More importantly, these tracers determine only the deoxyribonucleoside salvage pathway, which does not include the rate of the de novo synthesis of DNA starting from the formation of nucleotides, described as above. Studies have demonstrated that the deoxyribose (dR) of deoxy-adenosine (dA) and deoxyguanine (dG) is derived almost entirely from extracellular glucose, the reuse of free dR from degradated DNA does not occur, and that of the dA and dG is minor:1012 therefore, the de novo nucleotide synthesis pathway predominates in the synthesis of deoxyadenosine (dA) and deoxyguanosine (dG).13-15
Based on the above considerations, Zhang et al.16 used [15N]-glycine as a precursor to estimate the de novo synthesis rate of the nucleotide base and, simultaneously, [U-13C6]glucose for the de novo synthesis rate of purine nucleotide. [15N]-glycine incorporated into the newly synthesized nucleotide base,15 and the [U-13C6]glucose yielded 13C-labeled ribose,17 which subsequently formed the 13C-ribose-labeled nucleotide. 13C-ribose-labeled nucleotide is regarded as an approximation of total DNA synthesis.16,17 The difference between the synthesis rates of 13C-ribose-labeled deoxynucle-otide and the [15N]-labeled deoxynucleotide represents the nucleotide base salvage pathway.16 Thus, the investigators successfully measured the fractional rate of total DNA de novo synthesis of rabbit skin at approximately 3% per day, and they found that the rate of DNA synthesis starting from the de novo nucleotide base was 2.5% per day. Hence, about 0.5% (= 3% - 2.5%) DNA synthesis occurs via the base salvage pathway. The established animal and isotope model is a novel approach with which to quantify the metabolic marker of cell proliferation in the skin, simultaneously with the rate of protein synthesis, and the influence of whole body nutritional status under various physiological and pathophysiological conditions.
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