Determination of 35S-aminoacyl-transfer ribonucleic acid specific radioactivity in small tissue samples

Rate determination of protein synthesis utilizing tracer amino acid incorporation requires accurate assessment of the specific radioactivity of the labeled precursor aminoacyl-tRNA pool. Previously published methods presumably useful for the measurement of any aminoacyl-tRNA were unsuccessful when applied to [³⁵S]methionine, due to the unique chemical properties of this amino acid. Herein we describe modifications of these methods necessary for the measurement of ³⁵S-aminoacyl-tRNA specific radioactivity from small tissue samples incubated in the presence of [³⁵S]methionine. The use of [³⁵S]methionine of high specific radioactivity enables analysis of the methionyl-tRNA from less than 100 mg of tissue. Conditions for optimal recovery of ³⁵S-labeled dansyl-amino acid derivatives are presented and possible applications of this method are discussed.     

Some electrophysiological consequences of electrogenic sodium and potassium transport in cardiac muscle: a theoretical study

A previous paper described a kinetic model for electrogenic sodium-potassium transport in cardiac muscle, combining a thermodynamically-constrained transport model with simple passive permeabilities for sodium and potassium to generate a cardiac action potential (Chapman, Kootsey & Johnson, 1979). The present paper explores the extent to which this simplest of active-passive transport models can account (without further modification) for the electrophysiological behavior of cardiac muscle. The long term (several minutes) changes in the duration of the action potential observed following a change in stimulation rate are predicted by the model through a shift in the steady-state current-voltage relationship caused by small changes in inside ion concentrations. The diastolic hyperpolarization observed following an increase in rate is also predicted, including the linear relationship between the maximum diastolic depolarization and the rate of stimulation. Varying the outside potassium concentration in the model produces changes in the rest potential and current-voltage relationship similar to published data. Deviations from ideal potassium electrode behavior occur at both high and low concentrations because of effects on the pump. The model not only predicts the observed shift of the current-voltage curve in the depolarizing direction with increasing [K⁺]₀, but also the crossing of the curve in normal [K ⁺]₀ without having to assume a variation in g_K. Anoxia was introduced into the model by changing the concentrations of ATP and ADP, thereby enabling the model to account for the rapid diastolic depolarization observed in myocardial ischemia.     

Actin acetylation in Drosophila tissue culture cells

In a permanent cell line derived from Drosophila embryos, cytoplasmic actin is produced as an unstable precursor, which is subsequently converted to a stable form. This conversion results in a reduction in isoelectric point, with no apparent change in molecular weight. The conversion involves an enzymatic acetylation, and results in an insensitivity to aminopeptidase digestion, suggesting N-terminal blockage. Both the acetylated and unacetylated actins can participate in the assembly of F-actin, but with different efficiencies.This work was supported by a grant from the NIH (GM 22866).     

'Growing season' as a factor in sea trout production

Correlations between mean age at smoltiftcation (MSA) of sea trout post-smolts and length of growing season are presented. The first year of the two-year smolt phase is the more important but the second is also significant as is the year of migration (the third). Strongest correlations are for MSA and the total number of growing days in all three years. A continuous record of calculated MSA from 1916–71 suggests there should have been relatively larger bursts of sea trout production in the 1940s and between 1955 and 1965. The number of trout migrating from one catchment correlates significantly with the number of growing days in the two-year smolt phase.     

Mathematical model for enzymatic hydrolysis and fermentation of cellulose by Trichoderma

This paper describes a mathematical model for the enzymatic hydrolysis and fermentation of cellulose by Trichoderma reesei. The principal features of the model are the assumption of two forms of cellulose (crystalline and amorphous), two sugars (cellobiose and glucose), and two enzymes (cellulase and β-glucosidase). An inducer–repressor–messenger RNA mechanism is used to predict enzyme formation, and pH effects are included. The model consists of 12 ordinary differential equations for 12 state variables and contains 38 parameters. The parameters were estimated from four sets of experimental data by optimization. The results appear satisfactory, and the computer programs permit simulation of a variety of system changes.     

Cross-sectional area of flexible tubes

Kresch (1977) presented the results of calculations which determined the cross-sectional shapes assumed by flexible, elastic tubes for varying transmural pressures. Extensions of these results are presented here in the form of graphs of the cross-sectional area as a function of the transmural pressure. Since the circumferential arc length, theX-axis intercept and theY-axis intercept were necessarily computed, and are of interest, graphs of these are also presented. Each graph is a composite of several curves. Each curve in the graph shows the behavior of the parameter as a function of the relative pressure (P) for a specific value ofR, a parameter related to the wall thickness. The curves are shown for both positive and negative values ofP. The most negative value ofP shown is the point where the walls touch and the most positive value shown is the point where the equations fail to converge. The cross sections were assumed to be elliptical when the transmural pressure was zero. Graphs for three of these initial shapes are shown: nearly circular, major-to-minor axis ratio equal to 2, and an intermediate case. For the nearly circular case, the curves divide into two regimes: one where stretching clearly predominates and another where bending clearly predominates. For the more eccentric initial cross-sections, the distinction between the two regimes cannot be made in this simple manner.