DETECTION AND CHARACTERIZATION OF A PROTEIN ISOASPARTYL METHYLTRANSFERASE WHICH BECOMES TRAPPED IN THE EXTRACELLULAR-SPACE DURING BLOOD-VESSEL INJURY

Injury to rat blood vessels in vivo was found to release intracellular pools of protein D-aspartyI/L-isoaspartyl carboxyl methyltransferase (PIMT) into the extracellular milieu, where it becomes trapped. This t rapped cohort of PIMT is able to utilize radiolabeled S-adenosyl-L-met hionine (AdoMet) introduced into the circulation to methylate blood ve ssel proteins containing altered aspartyl residues. As further shown i n this study, methylated substrates are detected only at the specific site of injury. In vitro studies more fully characterized this endogen ous PIMT activity in thoracic aorta and inferior vena cava. Methylatio n kinetics, immunoblotting, and the lability of methylated substrates at mild alkaline pH were used to demonstrate that both types of blood vessel contain an endogeneous protein D-aspartyl/L-isoaspartyl carboxy l methyltransferase (PIMT). At least 50% of the PIMT activity is resis tant to nonionic detergent extraction, suggesting that the enzyme acti vity becomes trapped within or behind the extracellular matrix (ECM). Quantities of lactate dehydrogenase (LDH), another soluble enzyme of p resumed intracellular origin, were found to be similarly trapped in th e extracellular space of blood vessels.