A-Acetyl-Hydroxylation of Sialic Acids

A -Acetyl-Hydroxylation of Sialic Acids 4.4.1. Occurrence of A -GIycoIoylneuraminic Acid 

Kitajima et aL, 1988 Kanamori et aL, 1990), the presence of Neu5Gc in mammalian NCAM-associated polysialic acid has yet to be demonstrated (Troy, 1992). (See Chapter 4.) 

The hydroxylase in supernatants of various tissues exhibits an apparent for CMP-Neu5Ac ranging from 18 (xM for the enzyme from A. rubens (Schlenzka et al., 1993a) through to 2.5 (xM for the hydroxylase in pig submandibular glands (Muchmore et al., 1989), 1.3 xM for the mouse liver enzyme (Shaw and Schauer, 1989), and 0.6 (xM for the hydroxylase from rat small intestine (Bouhours and Bouhours, 1989). Although the complex substrate and cofactor 

Most NAD(P)H-dependent monooxygenases usually consist of two or three loosely associated protein components. These form an electron transport chain where an NAD(P)H oxidoreductase passes electrons to a terminal acceptor which catalyzes the reductive activation of oxygen and the subsequent hydroxylation of the substrate (Hayaishi, 1974). The inhibitory effect of increased ionic strength and dilution on the activity of the hydroxylase from mammalian sources points to the involvement of protein-protein interactions during catalysis (Shaw et al., 1992). This has been confirmed in a number of studies in which the participation of cytochrome b5 and cytochrome b5 reductase in the hydroxylase reaction of mammalian tissues was demonstrated (Kozutsumi et al., 1990 Shaw et al.. 

FIGURE 6. Redox protein components involved in the hydroxylation of CMP-Neu5Ac. 

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