5′‐Nucleotidase from the electric ray electric lobe

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Abstract
A cDNA encoding a 5′‐nucleotidase was identified by screening a λgt10 cDNA library from the electric lobe of Discopyge ommata using a cDNA probe containing the complete open reading frame coding for the rat liver enzyme. Nucleotide sequence analysis defines an open reading frame of 577 amino acids, corresponding to a calculated molecular mass of 63833 Da. The N‐terminus of the mature protein, as determined by direct protein sequencing, is preceded by 29 amino acid residues comprising a signal peptide. The C‐terminus contains a stretch of hydrophobic amino acids, considered to be cleaved on post‐translational modification and exchanged for glycosylphosphatidylinositol as a membrane anchor. The predicted protein contains four potential N‐linked glycosylation sites. Electric ray 5′‐nucleotidase shares 61% amino acid identity with the enzymes from rat liver and human placenta, and about 23% with bacterial proteins possessing 5′‐nucleotidase activity and also additional enzyme activities like UDP‐glucose hydrolase. Polyclonal antibodies raised against 5′‐nucleotidase from mammalian sources or the electric ray electric organ reveal mutual crossreactivity. Interestingly, there are 5–7 domains highly conserved in procaryotes and vertebrates in enzymes exhibiting 5′‐nucleotidase, 3′‐nucleotidase or phosphodiesterase activity. 5′‐nucleotidase isolated from Torpedo electric organ hydrolyzes UDP‐glucose at 8% of the rate of AMP hydrolysis. The possible phylogenetic origin of vertebrate 5′‐nucleotidase from multifunctional nucleotide hydrolases is discussed.

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