Cytidine triphosphate, enzymic preparation

The N-acetylneuraminic acid derivative 44 is widely distributed. It was isolated from a strain182 of Escherichia coli, and has been obtained from cytidine 5 -triphosphate and N-acetylneuraminic acid by the action of enzyme preparations from Neisseria meningitidis183 and from animal tissues.184-186 The latter enzyme can also make use of N-glycolylneuraminic acid as a substrate, to give the respective cytidine 5 -phosphate derivative. 

A similar approach has been used to prepare oligosaccharides that contain sialic acid (N-acetylneuraminic acid) (8). We saw earlier that sialic acid itself can be prepared using an aldolase reaction, and this forms the starting point for another cascade of enzyme-catalysed reactions. In this instance, sialic acid is firstly activated as its cytidine monophosphate (CMP) derivative via coupling with cytidine triphosphate (CTP) in the... 

On the basis of the early observation that the increases in microsomal enzyme activity produced by phenobarbital and 3-methylcholanthrene were blocked by actinomycin-D, it was suggested that enzyme induction resulted from the synthesis of new enzyme protein which was, in turn, dependent upon the DNA-directed synthesis of a messenger-like RNA. Treatment of rats with 3-methylcholanthrene causes an increase of about 40% in the level of RNA in rat liver nuclei and the nuclear RNA from 3-methylcholanthrene-treated rats is more active in directing protein synthesis than RNA from control animals. Moreover, the in vitro incorporation of radioactive precursors such as orotic acid or cytidine triphosphate into nuclear RNA is 50 to 100% greater in preparations from 3-methylcholanthrene-treated animals than controls. It is of interest that treatment of rats with phenobarbital has been recently reported to result in a marked suppression of endogenous hepatic ribonuclease activity. 

The kinase which converts deoxycytidine to its 5 -monophosphate has been studied most extensively in preparations from calf thymus 35, 36). The preferred substrate is deoxycytidine, for which the Michaelis constant (5 X 10 M) is much lower than that of two other substrates, deoxyadenosine and deoxyguanosine. Cytidine, uridine, and thymidine are not phosphorylated by this enzyme. Deoxycytidine kinase is subject to a complex pattern of allosteric regulation by nucleotides. The end product of deoxycytidine phosphorylation, dCTP, is a potent inhibitor this inhibition is reversed by dTTP. The enzyme has a rather broad specificity for the phosphate donor, with the triphosphates of the natural ribo- and deoxyribonucleosides being substrates the inactivity of dCTP is a notable exception. 

Whitesides and coworkers have carried out a comparison of enzymic and chemical routes to CTP, GTP and UTP on a 10-gram scale. They concluded that CTP and GTP were best made enzymically, and UTP by reaction of CTP with nitrous acid. The triphosphates were then employed for the enzymic synthesis of UDP-Glucose, UDP-Glucuronic acid, and GDP-Mannose.i94 Cytidine diphosphate sugars have been prepared from the 3,6-dideoxyhexoses paratose and abequose,193 and all four nucleoside diphosphate sugars of 6-sulpho-a-D-quinovose have been synthesized for studies of sulpholipid biosynthesis in chloroplasts.196 The stable analogue (138) of CMP-KDO has been prepared by a triester approach, but was only a weak inhibitor of KDO incorporation into lipopolysaccharides.197 A reference to acetylated forms of UDPGlc is mentioned in Chapter 7. 

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