Plasma ascorbic acid specific activity

Radioactivity. The specific activity in plasma, defined as 10 dpm of C-radioactivity/mg of unlabeled ascorbic acid, is plotted against dissection time in Figure 1. The time courses of C-radioactivity for various tissues are presented in Figures 2-4. 

Specific Activity. The time course of the specific activity of ascorbic acid in plasma has been fitted by an analog computer to a sum of four exponentials ... 

Si specific activity (10 dpm/mg) of ascorbic acid in plasma XT amount (10 dpm) of C-radioactivity in the organ Tit turnover rate (mg/h) of ascorbic acid from plasma into the organ... 

Consequently the total turnover in plasma can be obtained by introducing the plasma specific activity (Si, Equation 1) into Equation 9. The Ttot values calculated using Equation 9 are listed in Table III. These values may be compared with the daily intake of ascorbic acid present in the diet, which was determined to be 16 it 3 mg. 

Under the experimental conditions, the overall half-life (biological half-life) of ascorbic acid elimination from the body was calculated at about 61 h (2.5 d). This value has been evaluated from the slope of the log linear phase ( -phase), indicating equilibration of the labeled ascorbic acid with the exchangeable body pool. The observed half-life compares well with data obtained in earlier studies— mainly calculated from the time dependence of the logarithm of radioactivity remaining in the body, and with the assumption that metabolites of ascorbic acid are rapidly eliminated. Thus, biological half-lives for ascorbic acid of GO-MO h (3), 85-115 h (36), 48-72 h (2), or 127 h (22) are reported. In one study (22) the half-life was determined from the plasma specific activity to be only 39 h. 

Metabolic Roles. Ascorbic acid is an electron donor required for a variety of oxidative processes. It is readily regenerated by glutathione, NAD, and NADP and thus has a long biological half-life. Currently, there are eight known human enzymes that require ascorbic acid, and they are listed in Table 8.5. The precise metabolic roles have not been completely elucidated, but it appears that in the met-alloenzymes, ascorbate reduces the active metal site. In addition to these specific enzymes, ascorbic acid seems to function as a free-radical scavenger in the aqueous phase of plasma and cells. 

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