No abstract

The rotational motion of the sarcoplasmic reticulum Ca2 -activated ATPase (ATP phosphohydrolase, EC 3.6.1.3) has been investigated by measuring the decay of laser flash-induced dichroism with the covalently attached triplet probe eosin isothiocyanate. The Arrhenius plot for rotational mobility indicates two discontinuities at 415'C and -350C. The experimental data are rationalized in terms of a sudden conformeric change in the ATPase at 15'C and a temperaturedependent equilibrium existing between the conformationally altered ATPase and oligomeric forms of it in the temperature range 15-350C. The enzymatic activity, as indicated by a discontinuity in the Arrhenius plot for the rate of ATP hydrolysis, appears to be sensitive only to the change at 15'C. There is a strong correlation between the activation energy below 15'C for rotational motion (33.6 + 2.2 kcal/mol) and enzymatic activity (34 ± 4 kcal/mol). The concept of biomembrane fluidity put forward by Chapman and coworkers (1) has been a fruitful one for emphasizing the

No abstract

No abstract

The structure, chemical composition and function of the microsomal fraction, isolated by differential centrifugation and purified on sucrose gradients, from muscle of fetal, newborn and young rabbits were characterized and compared with those of sarcoplasmic reticulum vesicles from adult muscle. Negative staining shows that the microsomal vesicles isolated from muscles of embryos and newborn animals are smooth, in contrast to vesicles obtained from adult muscle which contain 4‐nm particles on their surface. The particles appear first in the microsomal vesicles from muscles of 5–8‐day‐old rabbits. Their number increases with the age of the animals. Ca2+‐pump protein, with molecular weight about 100000, accounts for 10% of the total protein content in sarcoplasmic reticulum membrane, isolated at the earliest stages of development analysed. Its amount increases continuously with the rabbit's age to the adult value of about 70% of total sarcoplasmic reticulum protein. The low amount of 100000‐dalton protein and lack of 4‐nm surface particles in sarcoplasmic reticulum vesicles obtained from fetal and newborn rabbits are strictly correlated with the low activity of Ca2+‐dependent ATPase and the ability to take up Ca2+. These activities rise in parallel with the age of the rabbits. On the other hand, Mg2+‐dependent ATPase activity is very high at the early stages of development and declines continuously to a low value in sarcoplasmic reticulum from adult muscle. The sarcoplasmic reticulum membrane from fetal and newborn rabbits contains a higher amount of lipids as compared with the membrane present in the muscle of adult animals. The ratio of both phospholipid to protein and neutral lipid to protein decreases with the age of the rabbits. The composition of sarcoplasmic reticulum phospholipid also changes during development.