Conversion of MM creatine kinase isoforms in human plasma by carboxypeptidase N

This study was undertaken to Identify the carboxypeptidase(s) (CPase) In plasma mediating sequential conversion of the tissue isoform of the MM isoenzyme of creatine kinase (MM, CK) to MM₂ and MM, Isoforms and to elucidate relationships between CPase activity measured in plasma and observed rates of isoform conversion in vitro. Purified MM, was incubated at 37 C in plasma from normal subjects and patlentswith acute myocardial infarction. Isoformswere quantified by chromatofocusing. Preincubation with antiserum to CPase N prevented conversion of added MM, to MM, and MM,. Isoform conversion rates In the absence of antibody were proportional to plasma CPase N activity assayed spectrophotometrically by hydrolysis of furylocryloyl-L-alanyl-L-lysine substrate (r = 0.89, n = 8). Plasma CPase N activity varied by nearly 300% among individuals, but average activity was similar in samples from normal subjects (267 ± 45 [SD] U/L, n = 18), those from outpatients with angina (289 ± 43 U/L, n = 9), and those obtained at hospital admission from patients with acute infarction (Q wave: 279 ± 70 U/L, n = 16; non-Q wave: 272 ± 61 U/L, n = 14) or unstable angina (280 ± 71 U/L, n = 11). In patients with A wave infarction, CPase N activity increased by 43% ± 25% between 48 hours and 72 hours (P < 0.005 compared with admission) with a concomitant change in the rate of conversion of isoforms. Thus, the rate of conversion of isooooms in individual subjects can be estimated by assay of CPase N activity in plasma. Accordingly, quantification of CPase N activity should permit compensation for individual variation in the rate of isoform turnover, thereby facilitating more accurate diagnosis and timing of the onset of infarction and of reperfusion based on analysis of MM CK isoform profiles in plasma.