Lysosomes 5’-nucleotidase

Nucleotidases have been studied in liver from various species and activity has been identified in lysosomes, cytoplasmic supernatants and plasma membrane preparations. Arsenis and Touster (31) have purified a 5 -nucleotidase from rat liver lysosomes to apparent homogeneity. The enzyme is unusual in that it hydrolyzes 2 -, 3 -, and 5 -mononucleotides equally well with preference for 5 -dAMP. It also hydrolyzes FMN, p-nitrophenyl phosphate, and /J-glycerol phosphate, but not inorganic pyrophosphate or bis(p-nitrophenyl) phosphate. Unlike the 5 -nucleotidases described thus far, divalent cations such as Co2+, Mn2+, and Mg2+ have no activating effect, but EDTA is inhibitory. In spite of the broad substrate specificity kinetic experiments indicate that a single enzyme is involved. Because of its broad substrate specificity it has been suggested (SI) that it may play a key role in lysosomal catabolism of nucleic acids. 

Thus, there is likely as many as three enzymes with 5 -nucleotidase activity in liver, one lysosomal, one cytoplasmic, and one membrane bound. Their specificities and kinetic properties appear to be distinctly different. This would suggest specialized physiological functions not yet understood. 

Figure 17 Facing page) Comparison of the subceUular distribution of HPMA copolymer-galactosamine (58) and cationic HPMA copolymers containing pendant side-chains terminating in oxyethy-trimethylammonium chloride (56), structure in panel (a). The profiles shown in panels (b) and (c) show rat liver fractionation using a percoll gradient at various times after iv administration of the conjugates. Panel (b) shows the distribution of I-labeled cationic HPMA copolymers at -e-10 min, -d- 20 min and -c- 60 min. Aiyl sul-fatase distribution (lysosomes) is shown in -f-. Panel (c) shows the distribution of I-labeled HPMA copolymer-galactosamine in this case, -g-10 min, -f- 20 min and -c- 60 min. Arylsulfatase distribution (lysosomes) is shown in -a- and 5 -nucleotidase (plasma membrane) in -C-.

The widely distributed and relatively nonspecific acid and alkaline phosphatases can convert nucleoside monophosphates to nucleosides, regardless of the base or of the position of the phosphate (2, 3, or 5 ). Rat liver lysosomes contain an acid nucleotidase which is not specific regarding the position of the phosphate, but which hydrolyzes adenine nucleotides faster than those of other bases (6) it is different from the nonspecific lysosomal sugar phosphatase. Some plants also contain a specific 3 -nucleotidase. 

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