Xenobiotic conjugates hidden

With few exceptions, diagrammatic presentations of metabolic pathways for xenobiotics include a number of structures that are enclosed within brackets. This is the means by which the otetabolism chemist designates a proposed intermediate, or hidden metabolite, in that particular pathway. Often, the proposed intermediate is considered so transient in nature that its isolation would be impractical, if not impossible, and its reactivity so great that its occurrence as a terminal metabolite would be virtually inconceivable. When secondary metabolic processes are involved, these transient metabolites are referred to as hidden conjugates. These intermediate, transitory metabolites have their own chemical and toxicological properties and it is reasonable to assume that they may influence, at least in part, the toxic action of the parent xenobiotic. 

One of the most intriguing facets associated with the study of xenobiotic metabolism is the occasional discovery of metabolites whose pathways of formation defy logical explanation. One of the least intriguing is the report by a colleague that a major metabolite was overlooked in one s own study. Each of these situations may arise from what might be termed "hidden" metabolites. These hidden metabolites are common to both primary and secondary biotransformation processes, but this paper deals solely with the latter where the metabolite is referred to as a hidden conjugate. 

Perhaps the most common reason for the existence of hidden conjugates, especially in the past, is the failure of the analytical methods employed to detect total xenobiotic residues present in a sample. This problem has been solved to a great extent by the use of radiotracer techniques in xenobiotic investigations. Once its existence is established, the analytical chemist can usually isolate and identify the metabolite. This, however, does not mean that... 

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