Creatine kinase isoenzymes in serum from cord blood and the blood of healthy full-term infants during the first three postnatal days.

Isoenzymes of creatine kinase (ATP:creatine phosphotransferase; EC 2.7.3.2; CK) were measured by electrophoresis in serum from cord blood and skin-puncture blood taken from 45 healthy full-term infants during the first three postnatal days. Mean total CK activities (in U/L at 30 degrees C) were 185 in cord samples, 536 in samples taken between 5--8 h postnatally, 494 between 24--33 h, and 288 in the 72-100 h samples. Values for all three isoenzymes increased to a peak over this period, with the highest values generally being found in the samples taken 5--33 h after birth; the subsequent decline was most rapid for CK-BB. Serum CK isoenzymes in cord samples and those taken at 72--100 h in the 11 babies delivered by cesarian section did not differ significantly from those of babies delivered vaginally. However the postnatal increases in total CK, CK-MM, and CK-MB (but not in CK-BB) were significantly greater in those patients born by vaginal delivery. The reasons for the increases in CK isoenzymes after birth are not clear, but our results and reported studies on the ontogeny of CK suggest that CK-MB cannot be regarded as a "cardiac-specific" isoenzyme in the neonatal period.