Identification of protein-ribulosamine-5-phosphatase as human low-molecular-mass protein tyrosine phosphatase-A

Ribulosamines, which are substrates for the deglycating enzyme fructosamine-3-kinase-related protein, are presumably formed intracellularly through glycation of proteins with ribose 5-phosphate followed by dephosphorylation of resulting RN5Ps (ribulosamine 5-phosphates) by a putative RN5Pase (ribulosamine-5-phosphatase). Ribose 5-phosphate is known to be a potent glycating agent and we show in the present study that it reacts approximately 10 and 80-fold more rapidly with protein than ribose and glucose respectively. We also show that tissue extracts and, most particularly, erythrocyte extracts contain a protein-RN5Pase. We have purified this enzyme from human erythrocytes to near homogeneity and shown it to correspond to LMWPTP-A [low-molecular-mass ('weight') protein tyrosine phosphatase-A]. Human recombinant LMWPTP-A displayed an RN5Pase activity that was higher than its tyrosine phosphatase activity, indicating that this phosphatase may participate in protein deglycation, a new form of protein repair.