Fig. 1.

Measuring the SRX in human fibers. The intensity of the fiber fluorescence, relative to the pre-chase value, is plotted as a function of time during the chase phase of the single nucleotide turnover measurements in the presence (blue circles) or absence (red squares) of 100 μM piperine. Human fibers were obtained from lean subjects. The fibers were first incubated in the fluorescent ATP analog, mantATP, and then chased with ATP. Fiber fluorescence decreases in two phases, a fast phase that is largely completed in roughly 20–30 s, followed by a slow phase with a lifetime of minutes. The slow phase arises from the slow release of nucleotides by myosin in the SRX. The slower phase has a decreased population in the presence of piperine, showing that the SRX has been partially destabilized. Fits to the data for control defined P2 = 0.31 ± 0.01, T2 = 157 ± 9 s; and with piperine P2 = 0.21 ± 0.01, T2 = 157 ± 12 s. Data averaged over a number of fibers is summarized in Table 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)