Chicken erythrocyte pyrimidine 5'-nucleotidase: purification and characterization of the subclass I enzyme

Erythroid pyrimidine 5'-nucleotidase: cloning, developmental expression, and regulation by cAMP and in vivo hypoxia

A characteristic process of terminal erythroid differentiation is the degradation of ribosomal RNA into mononucleotides. The pyrimidine mononucleotides can be dephosphorylated by pyrimidine 5'-nucleotidase (P5N-I). In humans, a lack of this enzyme causes hemolytic anemia with ribosomal structures and trinucleotides retained in the red blood cells (RBCs). Although the protein/nucleotide sequence of P5N-I is known in mammals, the onset and regulation of P5N-I during erythroid maturation is unknown. However, in circulating chicken embryonic RBCs, the enzyme is induced together with carbonic anhydrase (CAII) and 2,3-bisphosphoglycerate (2,3-BPG) by norepinephrine (NE) and adenosine, which are released by the embryo under hypoxic conditions. Here, we present the chicken P5N-I sequence and the gene expression of P5N-I during RBC maturation; the profile of gene expression follows the enzyme activity with a rise between days 13 and 16 of embryonic development. The p5n-I expression is induced (1) in definitive but not primitive RBCs by stimulation of beta-adrenergic/adenosine receptors, and (2) in definitive RBCs by hypoxic incubation of the chicken embryo. Since embryonic RBCs increase their hemoglobin-oxygen affinity by degradation of nucleotides such as uridine triphosphate (UTP) and cytidine triphosphate (CTP), the induction of p5n-I expression can be seen as an adaptive response to hypoxia.

Rabbit liver dCMP phosphohydrolase: a pyrimidine 5'-nucleotidase I-like enzyme in non-erythrocytic cells

A nucleotide phosphomonoesterase activity that preferably hydrolyzed dCMP was detected in rabbit liver and purified approximately 20-fold. The enzyme was similar in the catalytic and molecular properties to pyrimidine 5'-nucleotidase subclass I (P5N-I), which distributed specifically in vertebrate erythrocytes. In addition to liver, the activity was found in rabbit kidney, spleen, heart, intestine, but was not detected in any rat or chicken tissues tested. The rabbit enzyme protein reacted with antibodies against chicken P5N-I. Its pI was estimated to be approximately 5.3, and the enzyme was concluded to consist of single polypeptide of an approximately 38 kDa based on gel filtration and Western blot analysis. The partially purified enzyme preferentially hydrolyzes dCMP, UMP and CMP, K(m) values for these substrates are approximately 0.3 mM, the optimal pH is approximately 7, and the enzyme requires Mg(2+). This nucleotidase may contribute to the regulation of intracellular pyrimidine nucleotides in the rabbit.

Expression of pyrimidine 5'-nucleotidase subclass I during erythrocyte maturation in rats

The subclass I enzyme of rat pyrimidine 5'-nucleotidase (P5N-I), which preferentially hydrolyzes (deoxy)CMP and UMP, is distributed specifically in red blood cells (RBCs), and its activity increases approximately six-fold as compared to the control value after erythropoietic induction by phenylhydrazine administration. In this study, we detected rat P5N-I protein by using antibodies against the chicken P5N-I enzyme. The molecular mass of rat P5N-I was approximately 37 kDa, as estimated by gel filtration chromatography and Western blot analysis. The pI value of the enzyme was approximately 5.7. This protein band was detected only in RBC lysate extract, i.e., not in cytosol from the erythropoietic spleen. Protein mass of the P5N-I enzyme, estimated by immunoblot analysis, was increased in proportion to the enzyme activity after erythropoietic induction in rats. No phosphorylation of the enzyme protein was detected by immunoblot analysis with anti-phosphoserine or anti-phosphotyrosine antibody. In conclusion, these findings indicate that the rat P5N-I enzyme is expressed specifically in reticulocytes and may therefore be essential in the maturation process of rat erythrocytes.