Facts, noise and wishful thinking: muscle protein turnover in aging and human disuse atrophy.

Surprisingly little is known about the mechanisms of muscle atrophy with aging and disuse in human beings, in contrast to rodents, from which much has been extrapolated to explain the human condition. However, this extrapolation is likely unwarranted because the time course, extent of wasting, muscle fiber involvement and alterations of muscle protein turnover are all quite different in rodent and human muscle. Furthermore, there is little evidence that static indices of protein turnover represent dynamic changes and may be misleading. With disuse there are reductions in the rate of muscle protein synthesis (MPS) large enough to explain the atrophic loss of muscle protein without a concomitant increase in proteolysis. In aging, there is no evidence that there are marked alterations in basal muscle protein turnover in healthy individuals but instead the ability to maintain muscle after feeding is compromised. This anabolic resistance is evident with physical inactivity, which exacerbates the inability to maintain muscle mass with aging. The main conclusion of this review is that in uncomplicated, non-inflammatory disuse atrophy, the facilitative change causing loss of muscle mass is a depression of MPS, exacerbated by anabolic resistance during feeding, with possible adaptive depressions, rather than increases, of muscle proteolysis.

Location of translational initiation factor IF3 on the small ribosomal subunit.

The location of translational initiation factor IF3 bound to the 30S subunit of the Thermus thermophilus ribosome has been determined by cryoelectron microscopy. Both the 30S.IF3 complex and control 30S subunit structures were determined to 27-A resolution. The difference map calculated from the two reconstructions reveals three prominent lobes of positive density. The previously solved crystal structure of IF3 fits very well into two of these lobes, whereas the third lobe probably arises from conformational changes induced in the 30S subunit as a result of IF3 binding. Our placement of IF3 on the 30S subunit allows an understanding in structural terms of the biochemical functions of this initiation factor, namely its ability to dissociate 70S ribosomes into 30S and 50S subunits and the preferential selection of initiator tRNA by IF3 during initiation.

The activities of amylase and a trypsin-like protease in the gut contents of germ-free and conventional chickens.

1. Proteolytic and amylase activities were assayed in the gut contents of germ-free and conventional chickens aged 3 and 14 d. 2. The tendency was for the proteolytic activity to be increased and the amylase activity to be decreased in conventional chicks. These differences were statistically significant in the caecum. 3. All of the additional proteolytic activity detectable in the caecum of conventional chicks was inhibited by soybean trypsin inhibitor. However, the increased activity was not detectable in the small intestine and was, therefore, not due to pancreatic trypsin. 4. Attempts to demonstrate in vitro bacterial production of a trypsin-like enzyme were unsuccessful.

The antibiotic activity of the lactoperoxidase-thiocyanate-hydrogen peroxide system in the calf abomasum.

The lactoperoxidase system which had been previously shown to kill Gram-negative organisms (eg, coliforms, salmonellae, pseudomonads) in vitro, was found to be activated in vivo. The lactoperoxidase was provided by the milk and the thiocyanate either by the milk or by its secretion in the abomasum. The third factor was provided either by a H2O2 generating system (glucose oxidase and glucose) or by H2O2 producing lactobacilli. The latter occur naturally in large numbers in the abomasum of the calf.