SYNTHESIS AND BIOCHEMISTRY OF ASCORBIC ACID ANALOGUES

抗坏血酸(维生素C)是人类健康所必需的饮食，是几种酶作用的电子供体，具有抗氧化剂的功能，并与宿主防御机制、内分泌功能和视觉过程(晶状体)有关。最近，人们对抗坏血酸的生物化学重新产生了兴趣，因为人们意识到，人们对这几个角色发挥最佳作用所需的维生素浓度知之甚少。在酶反应的情况下，过程的最佳速率被定义为允许反应达到Vmax而没有毒性的浓度。作为确定这些浓度的计划的一部分，已经对某些与维生素C相关的反应进行了原位动力学测量。除了研究维生素C的功能作用外，最近对通过细胞膜转运维生素C的有效运输机制的表征强调了维生素在生物过程中的重要性。我们已经合成了放射性标记的6-脱氧-6-碘抗坏血酸，作为研究抗坏血酸运输系统的更多细节的工具。运输研究表明，这将是分离抗坏血酸运输蛋白的有用工具。为了研究2-羟基对抗坏血酸活性的重要性，我们以前制备了2-脱氧-抗坏血酸和2-脱氧-2-卤代抗坏血酸，包括2-脱氧-2-氟抗坏血酸，一种等位、等极、非氧化的类似物。还制备了2，2-二氟-2-脱氧抗坏血酸的环半酮型抗坏血酸，其结构与脱氢抗坏血酸的环半酮型相对应。分子模拟证实了2，2-二氟-2-脱氧抗坏血酸的环半酮形式和脱氢抗坏血酸的环半酮形式的相似性。正在研究这些类似物的转运特性，以及它们对将脱氧抗坏血酸还原为抗坏血酸的酶--戊二酸--的影响。6-卤代-6-脱氧-L-抗坏血酸类似物被氧化后，不能形成类似的环状半缩醛。如果环状结构是由脱氢抗坏血酸转运体运输的形式，则氧化的6-卤环类似物不应该被转移。对这个问题的研究正在进行中。基于亲核氟化物取代三氟化钠前驱体合成[18-F]-6-脱氧-6-氟-L-抗坏血酸的方法正在研究中。

Ascorbic acid (vitamin C), a dietary requirement for human health, is an electron donor for several enzymatic actions, functions as an antioxidant, and is implicated in host defense mechanisms, endocrine function and the visual process (lens). Recent renewed interest in the biochemistry of ascorbic acid has been prompted by the realization that relatively little is known concerning the concentrations of the vitamin required for optimum functioning of these several roles. In the case of enzymatic reactions, optimal rate of a process is defined as that concentration that allows the reaction to reach Vmax without toxicity. As part of a program to determine these concentrations, in situ kinetic measurements have been carried out for certain vitamin C-linked reactions. In addition to examination of functional roles of vitamin C, recent characterization of efficient transport mechanisms that translocate vitamin C across cellular membranes has emphasized the importance of the vitamin to biological processes. We have synthesized radiolabelled 6-deoxy-6-iodoascorbic acid as a tool for studying additional details of the ascorbic acid transport system. Transport studies indicate this will be a useful tool in attempts to isolate the ascorbic acid transport protein. To investigate the importance of the 2-hydroxyl group on ascorbic acid activity, we previously prepared 2-deoxy-ascorbic acid, and 2-deoxy-2-halo ascorbic acids, including 2-deoxy-2-fluoroascorbic acid, an isosteric and isopolar, non-oxidizable, analogue. The cyclic hemiketal form of 2,2-difluoro-2-deoxyascorbic acid also was prepared, a structure that corresponds to the cyclic hemiketal form of dehydroascrobic acid. Molecular modeling confirms the similarity of the cyclic hemiketal form of 2,2-difluoro-2-deoxyascorbic acid also the cyclic hemiketal form of dehydroascrobic acid. Transport properties of these analogues, as well as their effects on glutarodoxin, the enzyme that reduces deoxyascorbic acid to ascorbic acid, are being investigated. 6-Halo-6-deoxy-L-ascorbic analolgs, when oxidized, are unable to form a similar cyclic hemiacetal. If the cyclic structure is the form that is transported by the dehydroasorbate transporter, the oxidiazed 6-halo analogs should not be translocated. Studies to examine this question are in progress. Approaches to [18-F]-6-deoxy-6-fluoro-L-ascorbic acid are being investigated based on nucleophilic fluoride substitution of a triflate precursor.