Abstract
In muscle cells the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA) couples the free energy of ATP hydrolysis to pump Ca(2+) ions from the cytoplasm to the SR lumen. In addition, SERCA plays a key role in non-shivering thermogenesis through uncoupled reactions, where ATP hydrolysis takes place without active Ca(2+) translocation. Capsaicin (CPS) is a naturally occurring vanilloid, the consumption of which is linked with increased metabolic rate and core body temperature. Here we document the stimulation by CPS of the Ca(2+)-dependent ATP hydrolysis by SERCA without effects on Ca(2+) accumulation. The stimulation by CPS was significantly dependent on the presence of a Ca(2+) gradient across the SR membrane. ATP activation assays showed that the drug reduced the nucleotide affinity at the catalytic site, whereas the affinity at the regulatory site increased. Several biochemical analyses indicated that CPS stabilizes an ADP-insensitive E(2)P-related conformation that dephosphorylates at a higher rate than the control enzyme. Under conditions where uncoupled SERCA was specifically inhibited by the treatment with fluoride, low temperatures, or dimethyl sulfoxide, CPS had no stimulatory effect on ATP hydrolysis by SERCA. It is concluded that CPS stabilizes a SERCA sub-conformation where Ca(2+) is released from the phosphorylated intermediate to the cytoplasm instead of the SR lumen, increasing ATP hydrolysis not coupled with Ca(2+) transport. To the best of our knowledge CPS is the first natural drug that augments uncoupled SERCA, presumably resulting in thermogenesis. The role of CPS as a SERCA modulator is discussed.
Highlights
Ber of the P-type ATPase family [1]
CPS Activation of sarcoplasmic reticulum (SR) Ca2؉-ATPase (SERCA) and the Effect of Substrates—Results presented in Fig. 1A illustrate the effect of different concentrations of CPS on the steady state ATP hydrolysis by SERCA, measured at neutral pH
Part of the stimulation of SERCA activity by CPS was found to be independent on the presence of a Ca2ϩ gradient, as indicated by the mild stimulation of activity at concentrations of A23187 more than 1.5 g mlϪ1
Summary
Many studies in the medical literature have linked the consumption of chili-containing meals with increased energy expenditure and fat oxidation [26]. In this regard a seeming effect of chili consumption is the rapid increase in body core temperature and subsequent increased heat loss [27]. Our data are consistent with a model in which CPS increases the steady state concentration of a SERCA isomer in which occluded Ca2ϩ is released from an E2P-like form to the cytoplasmic side instead of the luminal side of the SR, resulting in uncoupling between ATP hydrolysis and Ca2ϩ transport. The increase in body metabolism and heat generation after CPS intake is likely a result of, at least in part, the direct effect of CPS on SERCA
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