Diversity in the sialic acids.

It is now more than 50 years since N-acetyl-neuraminic acid was first discovered and subsequently characterized by several groups (reviewed in Roseman, 1970; Gottschalk, 1972; Rosenberg and Schengrund, 1976; Schauer, 1982; Faillard, 1989). Relatively soon after its discovery, it became apparent that this molecule was actually the major member of a family of compounds related to neuraminic acid that were christened the 'sialic acids' (Blix etai, 1957). Early studies paid close attention to the different types of sialic acids and the interrelationships between them. However, interest in these complexities subsequently waned and unpublished 'folk-lore' had it that modified sialic acids were species-specific curiosities found only in a few tissues, such as erythrocytes and submaxillary glands. In fact, many investigators in the 1970s and 80s used the terms W-acetyl-neuraminic acid' and 'sialic acid' synonymously. Thus, for example, when structural or biological changes were noted following treatments with a sialidase (neuraminidase), it was often assumed that the sialic acid released was A^-acetyl-neuraminic acid. It is now clear tjiat the different types of sialic acids are much more widely distributed than previously thought. This review attempts to briefly summarize current knowledge concerning the occurrence, structure, biochemistry and biological significance of this diversity in the sialic acids. Particular attention is given to the two most common and better studied modifications: the addition of 0-acetyl esters to the hydroxyl groups at the 4-, 7-, 8and 9-positions, and the conversion of the 7V-acetyl group to an /V-glycolyl group. Given the breadth of the review, the bibliography is only representative, and tends to emphasize more recent studies.