On the presence of two non-specific nucleotide-sugar-hydrolysing enzymes in rat liver

Evidence is presented for the occurrence of two different non-specific nucleotide-sugar hydrolases in rat liver and other rat tissues. These two enzymes (I and II) were separated by chromatography on a 5'-AMP-aminohexyl-Sepharose column. Enzyme I is most probably identical with phosphodiesterase I (EC 3.1.4.1). Enzyme II appeared to be identical with an enzyme described in literature as 'CMP-sialic acid hydrolase' [Kean & Bighouse (1974) J. Biol. Chem. 249, 7813-7823], since almost all activity with CMP-N-acetylneuraminate as substrate was recovered in this enzyme fraction. CMP-N-acetylneuraminate was a poor substrate for Enzyme I, whereas deoxythymidine-5'-p-nitrophenyl phosphate and all nucleoside-diphosphosugars tested were good substrates for both Enzyme I and II. Therefore it is suggested that CMP-N-acetylneuraminate is used as an additional substrate to discriminate between the activities of Enzyme I and II in homogenates or membrane preparations. The various substrates appeared to be competitive inhibitors of each other, suggesting that, in each enzyme preparation, only one enzyme is responsible for the hydrolysis of the various substrates. The dissimilar properties of the two enzymes are substantiated by studying the subunit molecular masses (Enzyme I, 125 kDa; Enzyme II, 50-55 kDa), the sensitivity towards Triton X-100, Sarkosyl and sodium dodecyl sulphate and towards trypsin treatment. It is discussed whether the alpha-N-acetylglucosamine phosphodiesterase described by Varki & Kornfeld [(1981) J. Biol. Chem. 256, 9937-9943] is identical with one of the nucleotide-sugar hydrolases described here.