In the interest of full disclosure, the author wishes to inform the readers that he has been a paid consultant to the DiaSorin Company.
INTRODUCTIONVitamin D is a 9,10-secosteroid and is treated as such in the numbering of its carbon skeleton ( Fig. 47.1). Vitamin D occurs in two distinct forms: vitamin D 2 and vitamin D 3 . As shown in Figure 47.1, vitamin D 3 is a 27-carbon derivative of cholesterol; vitamin D 2 is a 28-carbon molecule derived from the plant sterol ergosterol. Besides containing an extra methyl group, vitamin D 2 differs from vitamin D 3 in that it contains a double bond between carbons 22 and 23. The most important aspects of vitamin D chemistry center on its cis-triene structure. This unique cis-triene structure makes vitamin D and related metabolites susceptible to oxidation, ultraviolet (UV)-light-induced conformational changes, heat-induced conformational changes, and attack by free radicals. As a rule, the majority of these transformation products have lower biological activity than vitamin D. It is important to note that, in humans, vitamins D 2 and D 3 provide similar potency (although some controversy exists as discussed in Chapter 61), and in this chapter the term vitamin D refers to both compounds.Metabolic activation of vitamin D is achieved through hydroxylation reactions at both carbon 25 of the side chain and, subsequently, carbon 1 of the A ring. Metabolic inactivation of vitamin D takes place primarily through a series of oxidative reactions at carbons 23, 24, and 26 of the side chain of the molecule. These metabolic activations and inactivations are well characterized and result in a plethora of vitamin D metabolites ( Fig. 47.2). Of the compounds shown in Figure 47.2, only four, vitamin D, 25-hydroxyvitamin D (25(OH)D), 24,25-dihydroxyvitamin D (24,25(OH) 2 D), and 1,25-dihydroxyvitamin D (1,25(OH) 2 D) have been extensively quantitated, and to date only two of those, namely, 25(OH)D and 1,25(OH) 2 D, provide any clinically relevant information. However, the quantitation of vitamin D and 24,25(OH) 2 D can provide important information in a research environment. Thus, this chapter addresses the quantitation of these four important vitamin D compounds. Further, it is not the intent of this chapter to address the detailed history of vitamin D metabolite analysis, as this can be obtained from CH 3 previous reviews [1e3]. Rather, the intent of this chapter is to describe how we currently measure vitamin D and its major metabolites in our laboratory, as well as to discuss the appropriate clinical judgments in the selection of a given compound for analysis.The first semiquantitative assay for vitamin D was a bioassay based on the rat-line test [4]. This assay was cumbersome, expensive, and relatively inaccurate. Real progress in vitamin D analysis was not achieved until the advent of high-specific-activity 3 H-labeled vitamin D 3 compounds [5]. The introduction of these tracers led to the development of competitive protein-binding assays (CPBA) for vitamin D and 25(OH)D [6,7]. A sho...