[Are children and adolescents less physically active today than in the past?]. / Er barn og unge mindre fysisk aktive i dag enn tidligere?

In recent years it has been claimed that Norwegian children and youth have reduced their level of physical activity and fitness. The aim of this systematic review was to investigate whether the amount of physical activity or level of physical fitness have changed among Norwegian children and youth during the past decades. We searched for relevant information in databases and reference lists and established contacts with scientists working in this field. The quality of the studies was evaluated on the basis of selection bias, information bias and adequate reporting of results. Five repeated cross-sectional studies of healthy boys and girls in the period from 1950 to 1997 were included. Three are based on tests of physical fitness and two of self-reported physical activity. The studies indicate that the physical fitness of the older boys has been reduced, the level of activity has been lowered, the percentage of inactive youth has increased and there are now larger variations in physical fitness. We conclude that there seems to be a negative trend in the level of physical fitness and physical activity of children and youth. This shows the need for interventions. The need for standardised parameters for physical activity is apparent.

Rates of different steps involved in the inhibition of protein synthesis by the toxic lectins abrin and ricin.

The kinetics of protein synthesis inhibition in a cell-free system from rabbit reticulocyte lysate was studied after addition of abrin and ricin and the isolated A chains. The toxin A chains inhibited protein synthesis at a rate proportional to the amount added. When intact toxins were added to the reticulocyte lysate, the kinetics of protein synthesis inhibition indicated that the A chains must be liberated before ribosome inactivation can take place. The splitting of the toxin in the lysate was directly demonstrated by the use of labeled toxins. The amount of abrin and ricin bound to HeLa cells under different experimental conditions was correlated to the concomitant inhibition of cellular protein synthesis. In the presence of lactose, which inhibits toxin binding, much higher concentrations of toxins were required to inhibit protein synthesis than in the absence of lactose. A linear relationship was found between the lactose concentration in the medium and the toxin concentration required to give 50% reduction in protein synthesis after 3 hours. The amount of toxin bound to the cell surfaces in the presence of lactose was either determined directly or calculated from the apparent association constant between toxins and surface receptors at the various lactose concentrations. Under different conditions involving a 300-fold variation in the concentration of toxin required to reduce protein synthesis by 50% after 3 hours, the amount of toxin bound to the cell surface was found to be the same. The toxicity thus appears to be determined by the number of toxin molecules bound to the cell surface. Purified ricin B chain was used to compete with the toxins for the receptor sites. Only after addition of high amounts of B chain was the toxicity of abrin and ricin appreciably reduced. The data do not support the view that receptors with especially high affinity are involved in the uptake of the toxins. When the time required for 50% inhibition was plotted versus the inverse value of the square root of the number of toxin molecules bound per cell, a straight line was obtained, intercepting at about 30 min. The data indicate that the observed lag time cannot be due entirely to the fact that the A chains must be liberated before they can act.

Introduction of B-chain-inactivated ricin into mouse macrophages and rat Kupffer cells via their membrane Fc receptors.

Experiments have been made to test whether the toxic lectin ricin can be bound to and introduced into cells by some other mechanism than via its B chain, the natural binding moiety of the toxin, without its toxic effect being neutralized. Complexes consisting of ricin and antibodies specifically directed against ricin B chain were incubated with mouse peritoneal macrophages and rat Kupffer cells, which are known to possess surface receptors for the Fc portion of the immunoglobulin molecule. After incubation for 26 h, cellular protein synthesis, as measured by incorporation of labeled leucine into acid-insoluble material, was completely inhibited. HeLa cells, which do not possess Fc receptors, were unaffected by the complex. The effect of the complex on protein synthesis of macrophages was prevented by soluble antigen-antibody complexes, but not by the presence of lactose which prevents attachment of the ricin B chain to the cell membrane. The [ricin-antiricin B] complex was attached to red cells, and the resulting complex was incubated with rat Kupffer cells. Cellular protein synthesis ceased after 6 h, and phase contrast microscopy studies showed that the complexes were taken up by the Kupffer cells. The data indicate that ricin, when present in the complex with antiricin B, can be introduced into cells through cell membrane receptors other than the B chain receptor, in this case the Fc receptor, and that the internalized toxin retains a least part of its activity.

Studies on the structure and properties of the lectins from Abrus precatorius and Ricinus communis.

The amino acid composition of the isolated A- and B-chains of the toxic lectins abrin and ricin was determined and compared. Even though the two toxins originate from widely different plants, statistical analysis of the amino acid content indicates extensive homologies in the amino acid sequence of the 4 chains. The intact lectins contain no free SH-groups whereas the isolated A- and B-chains contain close to one free SH-group each. The results indicate that in both toxins the A- and B-chains are connected by a single S-S bond. The B-chains of abrin and ricin contain similar amounts of mannose and glucosamine. The A-chain of ricin also contains some carbohydrate, whereas the A-chain of abrin appears not to be a glycoprotein. The non-toxic abrus and ricinus agglutinins contain more carbohydrate than abrin and ricin. The isoelectric points of the different lectin preparations were measured by isoelectrofocusing. The intact lectins are much more resistant to heat, freezing and chemical treatments than the isolated A- and B-chains. The intact lectins are also very resistant to treatment with proteolytic enzymes, whereas the isolated chains are easily digested. Evidence indicating that the toxins and their chains undergo extensive conformational changes upon reduction of the S-S bond is discussed.