Bending stiffness of lipid bilayers: IV. Interpretation of red cell shape change.

Two mechanisms are operative when the resting shape of human red cells is changed into an echinocyte or a stomatocyte. The first (bilayer couple) is a differential change in the surface area of the two monolayers. It rests on the two-dimensional isotropic elasticity of the two monolayers and their fixed distance. The second (single layer) is a change in the average cone angle of the molecules comprising a monolayer. It rests on the intrinsic bending elasticity of each single layer. With a few exceptions the first mechanism has been quoted to interpret experimentally observed shape changes. To reconsider this preference two types of spontaneous curvatures (in bilayer couple bending and in single-layer bending) are defined. It is shown that (a) disregarding the single-layer mechanism is not justified and (b) there is too little basic information for quantitative interpretations of shape change.     

pH-controlled continuous cultivation of mycoplasmas.

The continuous cultivation of mycoplasmas in a pH-controlled metabolistat was investigated with the fermentative strain Mycoplasma mobile 163K and the nonfermentative strain Mycoplasma arthritidis ISR1. The addition of medium and the removal of culture suspension were regulated by acid production from glucose by M. mobile 163K and by ammonium production from arginine by M. arthritidis ISR1, respectively. For both strains the optimal pH for continuous growth was 7.0. The steady state could be maintained for at least 21 days. With CFU of 8.4 X 10(9) ml-1 (M. mobile 163K) and 3.2 X 10(9) ml-1 (M. arthritidis ISR1), the cell concentrations were slightly higher than those obtained in batch cultures. The dependence on the adjusted pH values was measured for several parameters, such as flow rate, CFU, glucose fermentation or production of ammonia, and gliding velocity. Since the long lag phases of batch cultures can be avoided, pH-controlled continuous cultures provide an appropriate system for the production of mycoplasma cells.     

Regulation of peptide bond cis/trans isomerization by enzyme catalysis and its implication in physiological processes

In some cases, the slow rotational movement underlying peptide bond cis/trans isomerizations is found to control the biological activity of proteins. Peptide bond cis/trans isomerases as cyclophilins, Fk506-binding proteins, parvulins, and bacterial hsp70 generally assist in the interconversion of the polypeptide substrate cis/trans isomers, and rate acceleration is the dominating mechanism of action in cells. We present evidence disputing the hypothesis that some of the molecular properties of these proteins play an auxiliary role in enzyme function.