Koshiishi I, Mamura Y, Liu J, Imanari T
Faculty of Pharmaceutical Sciences, Chiba University, Japan.
Clin Chem. 1998 Apr;44(4):863-8.
The most popular pretreatment method of plasma samples for the measurement of ascorbate (AsA) and dehydroascorbate (DHA) has been an acidic deproteinization via metaphosphoric acid or trichloroacetic acid. In general, DHA is absent in plasma samples prepared from human blood in a conventional manner. However, when these plasma samples were subjected to acidic deproteinization, DHA was detected in the acidified sample solutions. In the present study, we demonstrate that the oxidation of AsA to DHA in the solutions was promoted by at least two mechanisms, one involving catalysis by ferric ion released from transferrin, and the other involving catalysis by plasma hemoglobin. In the acidified transferrin solution by trichloroacetic acid, an oxidation of AsA to DHA proceeded with standing time, whereas the oxidation was not observed in that by metaphosphoric acid. This oxidation appeared to be catalyzed by ferric ion released from transferrin. In contrast, plasma hemoglobin functioned as a catalyst for AsA oxidation in both metaphosphoric acid and trichloroacetic acid solutions. Therefore, DHA content in the trichloroacetic acid-treated plasma sample was markedly higher than that in the metaphosphoric acid-treated one. These results suggest that DHA detected in acidified plasma samples is an artifact resulting from AsA oxidation.
用于测定抗坏血酸（AsA）和脱氢抗坏血酸（DHA）的血浆样本最常用的预处理方法是通过偏磷酸或三氯乙酸进行酸性脱蛋白。一般来说，以常规方式从人血制备的血浆样本中不存在DHA。然而，当这些血浆样本进行酸性脱蛋白处理时，在酸化的样本溶液中检测到了DHA。在本研究中，我们证明溶液中AsA氧化为DHA至少通过两种机制促进，一种涉及转铁蛋白释放的铁离子催化，另一种涉及血浆血红蛋白催化。在经三氯乙酸酸化的转铁蛋白溶液中，AsA氧化为DHA随静置时间而进行，而在经偏磷酸酸化的溶液中未观察到这种氧化。这种氧化似乎由转铁蛋白释放的铁离子催化。相反，血浆血红蛋白在偏磷酸和三氯乙酸溶液中均作为AsA氧化的催化剂。因此，经三氯乙酸处理的血浆样本中的DHA含量明显高于经偏磷酸处理的样本。这些结果表明，在酸化血浆样本中检测到的DHA是AsA氧化产生的假象。
