Abstract
The influence of piperine on the enzymes and bioenergetic functions in isolated rat liver mitochondria and hepatocytes was studied. Piperine at lower concentrations (<50 μM) did not affect the RCR and ADP:O ratios, state 4 and 3 respirations supported by site‐specific substrates, viz. glutamate + malate, succinate, and ascorbate + TMPD. The site‐specific effects became significantly apparent only at higher concentrations. Only the state 3 respiration supported by NAD‐linked substrates was impaired equipotently in mitochondria and permeabilized hepatocytes; the effect appeared to be localized at energy‐coupling site 1. In hypotonic treated mitochondria, respiration supported by three kinds of substrates was not affected.
Among the respiratory chain‐linked enzymes, the activity of NADH‐dehydrogenase registered a significant decrease of about 25, 42, and 53% at 100, 150, and 180 μM piperine, respectively. The activity of Mg^++^‐ATPase, however, was stimulated at concentrations above 150 μM. Among the matrix enzymes, only malate and succinate dehydrogen‐ases were studied. Malate dehydrogenase only showed a strong concentration‐related inhibition in both the forward and backward directions. Enzyme kinetics indicated noncompetitive inhibition with a very low Ki of 10 μM. The presence of unsaturated double bonds in the side chain of piperine appeared essential for producing this strong inhibition. The studies suggested that piperine produces concentration related site‐specific effects on mitochondrial bioenergetics and enzymes of energy metabolism.