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Studies have been carried out to assess maturation of myofibrillar and mitochondrial proteins in fetal (1 13 to 140 days gestation), neonatal (-10 min to 21 days postpartum), and adult sheep hearts.Ca++-activated myosin ATPase activity was-20% lower in fetal than in adult left ventricular myocardium (1.13 + 0.06, n = 12, versus 1.36 .+ 0.07, n = 9, pmoles PI per g protein per sec; P < 0.025). In fetal and neonatal hearts (but not in adult hearts), myosin ATPase activity was slightly higher (-14%; P < 0.001) in right ventricular tissue than in left ventricular tissue. In contrast to these small changes in myosin ATPase activity, large changes indicative of maturation of energy metabolism occurred in the creatine kinase system: between 115 days gestation and 21 days postpartum, total creating kinase activity increased nearly &fold (0.2 to 1.6 IU/mg cardiac mass), the MM-creatine kinase isozyme increased 7-fold (0.2 to 1.5 IU/mg wet weight), and mitochondrial creatine kinase increased more than 25-fold (< 0.01 to 0.27 IU/ mg wet weight). The total creatine pool, but not the ATP pool, increased (from-6 to-15 nmoles/g tissue). Neither the concentration nor isozyme distribution of lactate dehydrogenase, a glycolytic enzyme, changed during this 7-wk period of development. Speculation and lactate dehydrogenase isozyme compositions. The results suggest that an age dependency exists in the enzymes important in regulating energy production and utilization. MATERIALS AND METHODS Myocardial tissue was obtained after rapid cardiectomy from 17 fetal sheep ranging in age from 113 to 140 days gestation, 22 newborn lambs ranging in age from 30 min to 21 days, and 13 adult (mother) sheep. ENZYME ANALYSES Myosin. Myosin (ATPase, EC 3.6.1.3) was isolated from 10 to 20 g tissue using the method of Katz (21). In all cases, the yield was-8 mg/g tissue. Calcium-activated myosin ATPase activity was assayed at 25OC in 0.1 M Tris (pH 8) containing 0.6 M KCI. 10 mM calcium, and 3.1 mM MgATP. The reaction was stopped by adding an aliquot of 70% perchloric acid. The liberated inorganic phosphate was measured using the Fiske-Subbarrow method (9). Myosin ATPase activity was calculated as moles of inorganic phosphate produced per protein per sec. The purity of the myosin preparations was assessed using sodium dodecyl sulfatepolyacrylamide gel electrophoresis (5% cross-linked) as previously described (15). We speculate that changes in biochemical composition of en-Separate tissue samples were used for analyses of creatine kinase ergy-utilizing and-producing proteins, coupled with alterations in and lactic dehydrogenase activities. Samples were homogenized the structure and cellular distribution of mitochondria and myofi-in 0.1 M potassium phosphate buffer (pH 7.4) with I mM EDTA brils, contribute to the changes in mechanical performance char-and 1 mM /I-mercaptoethanol, and allowed to incubate for I hr to acteristic of the maturing heart. release mitochondria1 isozyme as described by Jacobus and Lehninger (18). Creatine kinase activity (ATP; creat...

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