ASSOCIATION OF PURIFIED SKELETAL-MUSCLE AMP-DEAMINASE WITH A HISTIDINE-PROLINE-RICH-GLYCOPROTEIN-LIKE MOLECULE

Denaturation of rabbit skeletal-muscle AMP deaminase in acidic medium followed by chromatography on DEAE-cellulose in 8 M urea at pH 8.0 all ows separation of two main peptide components of similar apparent mole cular mass (75-80 kDa) that we tentatively assume correspond to two di fferent enzyme subunits. Whereas the amino acid composition of one of the two peptides is in good agreement with that derived from the nucle otide sequence of the known rat and human AMPD1 cDNAs, the second comp onent shows much higher contents of proline, glycine and histidine. N- Terminal sequence analysis of the fragments liberated by limited prote olysis with trypsin of the novel peptide reveals a striking similarity to the fragments produced by plasmin cleavage of the rabbit plasma pr otein called histidine-proline-rich glycoprotein (HPRG), However, some divergence is observed between the sequence of one of the fragments l iberated from AMP deaminase by a more extensive trypsinization and rab bit plasma HPRG in the region containing residues 472-477. A fragment with a blocked N-terminus, which was found among those liberated by pr oteolysis with pepsin of either whole AMP deaminase or the novel compo nent of the enzyme, shows an amino acid composition quite different fr om that of the N-terminus of the known subunit of AMP deaminase. By co upling this observation with the detection in freshly prepared AMP dea minase of a low yield of the sequence (LTPTDX) corresponding to that o f HPRG N-terminus, it can be deduced that in comparison with HPRG, the putative HPRG-like component of AMP deaminase contains an additional fragment with a blocked N-terminus, which is liberated by a proteolyti c process during purification of the enzyme. The implications of the a ssociation to rabbit skeletal-muscle AMP deaminase of a HPRG-like prot ein species are discussed.