Mechanical and biochemical cycles in muscle contraction

The current biochemical approach to understanding the mechanism by which chemical energy is converted into mechanical energy in muscle is by investigating the kinetics of the reaction of ATP with the isolated contractile proteins in solution with stopped and quenched flow techniques. The results are interpreted in terms of a cycle of hydrolysis of ATP. The current mechanical approach lies through the kinetics of force production or length change in preparations with the contractile proteins intact, by following step changes in length or tension. The results are usually interpreted in terms of mechanical cycles of activity of cross-bridges, in which a cross-bridge progresses through a series of states with different mechanical properties. If the biochemist's and the mechanist's cycles are one and the same, a problem arises over the correspondence between states in the two cycles. One approach to answering this question is through the determination of the effect of pH and of such substances as ATP, ADP and inorganic orthophosphate on the mechanical response. We hope that these will act at known points in the biochemist's cycle and their influence upon the mechanical cycle should enable their location here to be ascertained. The paper deals with the effects of phosphate, ATP and magnesium ions on the mechanical response of insect flight muscle.

Aberrant expression of TopBP1 in breast cancer

AIMS

The TopBP1 protein includes eight BRCT domains (originally identified in BRCA1) and has homology with BRCA1 over the carboxyl terminal half of the protein. The aim of this study was to determine whether TopBP1 is aberrantly expressed in breast cancer.

METHODS AND RESULTS

Sixty-one breast carcinomas from an unselected consecutive patient cohort were studied along with 12 samples of breast tissue from cosmetic breast reduction surgery; these were analysed immunohistochemically for TopBP1 expression using a rabbit polyclonal antibody. This antibody was validated in immunoprecipitation and immunofluorescence experiments. Immunohistochemical analysis demonstrated that TopBP1 was expressed almost exclusively in the nuclei of the normal breast epithelium. However, in a significant number of breast carcinomas TopBP1 was aberrantly expressed, as it was detected in the cytoplasm and nucleus of some tumours and exclusively in the cytoplasm of others. In two out of 61 carcinomas investigated, no TopBP1 expression was detected.

CONCLUSIONS

For the first time this report demonstrates aberrant expression of the TopBP1 protein in breast carcinoma. We propose TOPBP1 as a breast cancer susceptibility gene.

Calcaneal ultrasound attenuation in an elderly population: measurement position and relationships with body size and past fractures

This study demonstrates, the relationship between past fracture, body size and broadband ultrasound attenuation (BUA) and investigates two sites of BUA measurement in a representative elderly population of men and women (n = 2106). We measured BUA at a fixed position and at a consistent anatomic position within the calcaneus. We found fixed BUA was less closely correlated with stature and age than anatomic BUA. Both correlations were substantially weaker in men than in women. Mean BUA was significantly lower in women with a past fracture compared with nonfracturers (fixed BUA 63.3 vs 69.4 dB/MHz, p = 0.0004; anatomic BUA 77.6 vs 81.7 dB/MHz, p = 0.013). However, in women, the fixed BUA was better than the anatomic BUA at discriminating between fracturers and nonfracturers (OR 1.38/SD (95% CI 1.12-1.68) and OR 1.22/SD (0.99-1.52), respectively) when adjusted for body size and age. There was no significant difference in either BUA in men with or without a past fracture. In conclusion, currently the fixed position for BUA measurement is preferable and, whilst we have demonstrated that it is possible to locate an anatomically consistent point in the calcaneus, the position chosen by this study did not provide a measurement with more discriminatory capability than the fixed position. In women, BUA behaves similarly to bone mineral density in relation to stature and in its strength of association with past fracture, while the lack of association in men may reflect differing contributions by bone strength to fracture risk in the sexes.