An explicit relativistic light-front quark model is presented which gives the momentum transfer dependent form factors of weak hadronic currents among heavy pseudoscalar and vector mesons in the whole accessible kinematical region 0 ≤ q² ≤ q²_max. It is shown that in the limit of infinite masses of active quarks these form factors can be expressed in terms of universal Isgur- Wise function. The explisit expression for this function is obtained. It is shown that neglect of pair creation from the vacuum in calculations of form factors does not violate Luke's theorem.