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Biochemical evidence for Mn2+-dependent 5'-nucleotidase activity in isolated rod outer segments
1Department of Anatomy, University of Toronto, Ontario, Canada.
Abstract:
Recently cytochemical evidence has been presented for a novel enzyme activity, i.e. 'manganese-dependent pyrimidine 5'-nucleotidase (MDPNase)' activity in the rod outer segments (ROS) of rat retinas in situ and in isolated rat ROS. The present biochemical study was undertaken to seek further evidence for this enzyme activity using an independent method. A series of enzyme assays was carried out to test for MDPNase activity in Triton extracts of rat ROS isolated by sucrose density gradient centrifugation. Hydrolysis of the substrate, cytidine-5'-monophosphate, was measured spectrophotometrically and expressed as microgram of released inorganic phosphorus hr-1 mg-1 protein in the sample. The results showed that the ROS extracts contained enzyme activity (18.1 +/- 3.8) that was increased 5-6-fold (102.3 +/- 10.6) in the presence of 7.4 mM MnCl2. The enzyme activity was not enhanced by Mg2+ ions (19.0 +/- 7.7) and was strongly inhibited by 10-20 mM NaF (11.8 +/- 2.9). Assays for substrate specificity revealed that the Mn2+-stimulated phosphatase activity was specific for 5'-nucleotides. Pyrimidine nucleotides (5'-CMP and 5'-UMP) were the preferred substrates. Comparison of enzymatic hydrolysis of 5'-CMP and 5'-AMP over a pH range from 4.5 to 8.0 revealed that at acid pH, the majority of the observed 5'-nucleotidase activity (82% at pH 5.0, 58% at pH 5.5) was manganese dependent, whereas at neutral pH and above, most of the enzyme activity was unaffected by the presence of Mn2+ ions.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Researchers confirmed manganese-dependent pyrimidine 5'-nucleotidase (MDPNase) activity in rat retinal rod outer segments. This enzyme specifically hydrolyzes pyrimidine nucleotides, with activity varying by pH and manganese ion concentration.
Area of Science:
- Biochemistry
- Retinal Physiology
- Enzymology
Background:
- Cytochemical evidence suggested a novel manganese-dependent pyrimidine 5 omino-nucleotidase (MDPNase) in rat retinal rod outer segments (ROS).
- Further biochemical investigation was needed to independently confirm this enzyme activity.
Purpose of the Study:
- To biochemically characterize the manganese-dependent pyrimidine 5 omino-nucleotidase (MDPNase) activity in isolated rat retinal rod outer segments (ROS).
Main Methods:
- Enzyme assays were performed on Triton extracts of ROS isolated via sucrose density gradient centrifugation.
- Substrate hydrolysis (cytidine-5 omino-monophosphate) was measured spectrophotometrically.
- Enzyme activity was assessed in the presence and absence of Mn2+ and Mg2+ ions, and its substrate specificity and pH dependency were determined.
Main Results:
- ROS extracts exhibited significant enzyme activity, which increased 5-6 fold in the presence of 7.4 mM MnCl2.
- The enzyme activity was specific for 5 omino-nucleotides, with pyrimidine nucleotides (5 omino-CMP, 5 omino-UMP) being preferred substrates.
- Manganese-dependent activity was prominent at acidic pH (e.g., 82% at pH 5.0) and decreased at neutral and alkaline pH.
Conclusions:
- Biochemical assays provide strong evidence for manganese-dependent pyrimidine 5 omino-nucleotidase (MDPNase) activity in rat retinal ROS.
- The enzyme's substrate specificity and pH-dependent manganese requirement are key characteristics.
- This enzyme likely plays a role in nucleotide metabolism within the photoreceptor outer segments.