The neutrino mass mechanisms of the neutrinoless double beta decay (0νββ-decay) generated within the left-right symmetric models are discussed by taking advantage of an interpolating formula for nuclear matrix element depending on the neutrino mass. Its application is justified by a weak dependence of mean square neutrino momentum, which is a properly scaled ratio of nuclear matrix elements for the light and heavy neutrino mass mechanisms, on decaying isotope. Due to this fact there is a single unified lepton number violating parameter including dependendence on all neutrino masses in the decay rate and the light and heavy neutrino mass mechanisms can be hardly distinguished by observing the 0νββ-decay on different isotopes. Further, a see-saw type neutrino mixing, which relates neutrino mixing in light and heavy neutrino sectors, is proposed. By considering the current limit on the 0νββ-decay half-life of 136Xe the regions of dominance of exchange of light and heavy neutrinos are identified.