Nonlinear dependence on actin of acto-heavy meromyosin 18O exchange and ATPase rate at low actin concentrations

Hydrolysis of highly enriched [gamma-18O]ATP in unlabeled water by acto-heavy meromyosin at low actin concentration was found to be heterogeneous in that significant amounts of both the species containing 0 or 3 18O-labeled oxygens/phosphate were formed. Detailed quantitative comparison with theoretical distributions over a wide range of actin concentrations, however, indicated that the pathway which catalyzed ATP hydrolysis with a low extent of exchange only made a significant contribution at a low actin concentration and did not represent a major fraction of the total hydrolysis seen at higher actin concentrations. This low exchange component was also detected in the dependence on actin of the steady state ATPase. At low actin the steady state ATPase rate increased more rapidly as a function of actin concentration than predicted by the Km and Vmax for actin activation observed at moderate to high actin levels. This extra ATP hydrolysis at low actin correlates with that predicted for the low exchange pathway both with respect to the fraction of the ATP hydrolyzed and to its dependence on the actin concentration.     

A partial differential equation model of changing sizes and numbers in a cohort of juvenile fish

Synopsis Numbers and lengths of young crappie were measured over a 20-week period in a Tennessee River reservoir. From these data, the average length at the swimming interval and growth rate were computed. A partial differential equation model was formulated for changes in length distribution through time. The equation was solved analytically and this solution used to predict the numbers by length class through time, given an initial known recruitment or swim-up rate. The simulated numbers agreed in considerable detail with the observations, indicating that the model accurately reflected the basic population mechanics involved. This suggests the model can be used to predict impacts to the fish population of size-dependent mortality caused by man, such as impingement on power plant intake screens.