Mechanical Deformability of Biological Membranes and the Sphering of the Erythrocyte

Equations of mechanical equilibrium are applied to the erythrocyte membrane in the normal, hypotonically swollen, and sphered configurations. The hydrostatic pressure drop across the normal cell membrane is shown to be zero for all biconcave shapes if the membrane thickness is uniform. This result leads to the conclusion that the membrane tension is uniform and is a function of membrane potential. A two-dimensional fluid film model for the membrane is introduced to describe the unusual deformability of the erythrocyte during sphering in hypotonic solutions. The model predicts a smooth transition from the biconcave shape to a perfect sphere.     

Inducible degradation of hydroxyproline in Pseudomonas putida: pathway regulation and hydroxyproline uptake

Studies in Pseudomonas putida of the inducible degradation of hydroxyproline to alpha-ketoglutarate have indicated that either of the two epimers, hydroxy-l-proline or allohydroxy-d-proline, acts as an inducer of all the pathway enzymes. In a mutant lacking the first enzyme of the sequence, hydroxyproline-2-epimerase, which interconverts these two hydroxyproline epimers, either epimer is still equally active as an inducer of the remaining three enzymes, suggesting that each epimer has intrinsic inducer activity. The second and third enzymes of the sequence were induced coordinately. The induction process appeared to be insensitive to catabolite repression under a number of experimental conditions. The induced enzymes were stable even under conditions of nitrogen starvation and other conditions designed to increase protein turnover. In addition to inducing the degradative enzymes, the two hydroxyproline epimers were also found to induce an uptake system that concentrates hydroxyproline intracellularly. Either amino acid induced the uptake system for its epimer as well as for itself.     

Proteolysis and myelin breakdown: a review of recent histochemical and biochemical studies

Synopsis Proteins are important constituents of the myelin sheath and serve to maintain its structural integrity. One of the protein components is susceptible to tryptic digestion and may be regarded as a particularly vulnerable part of the myelin sheath. The initial events in myelin breakdown may involve disruption of lipid-protein attachments followed later by chemical degradation of released lipids.In Wallerian degeneration the activity of proteolytic enzymes increases by 12 hr after nerve section. Proteolytic enzyme activity increases in the prodromal phase of diphtheritic neuropathy. Extracts of degenerating nerve cause proteolysis of normal myelin with loss of trypanophilic basic protein and lipid; selective loss of basic protein occurs very early in Wallerian degeneration and has also been found in and around plaques of multiple sclerosis. Proteolytic activity is increased at the edges of active multiple sclerosis lesions. It has been shown that the basic encephalitogenic protein is susceptible to digestion by neural proteases, yielding an active encephalitogenic fragment.It is inferred from these collective observations that proteases play an important role in early myelin breakdown and may also be implicated in the pathogenesis of multiple sclerosis plaques by digesting basic protein, by releasing lipid from its attachment to such protein, and by liberating an active encephalitogenic peptide. The factors responsible for the activation and release of proteases remain unknown.Research Associate supported by the Multiple Sclerosis Society.