Saturation transfer in living systems

Abstract

N.m.r. studies of living systems can be used to obtain kinetic rates in vivo , in addition to providing information about metabolite levels and their time dependences. This is possible through the use of magnetization transfer techniques, which rely on the fact that a nuclear spin will remain in a given quantum state for a period of the order of its spin lattice relaxation time, T 1 , thus enabling perturbation of the nuclear spins in one molecular species and then observation of the transfer of that perturbation to another species, provided that the transfer takes place in a time of the order of T 1 . These times are about 1s in most systems, thus allowing the measurement of rates in the range of 0.1-10 -1 s by this method. These techniques were introduced by Forsen & Hoffman in a series of papers exploring their use in relatively simple chemical systems (Forsen & Hoffman 1963, 1964; Hoffman & Forsen 1969). An early biological application was to cytochrome c, by Gupta & Redfield (1970). The techniques have been applied to Escherichia coli to obtain an ATP synthesis rate (Brown et al . 1977) and to frog muscles and perfused rat hearts to obtain the exchange rates of ATP and phosphocreatine (PCr), this reaction being catalysed by creatine phosphokinase (CPK) in these systems (Brown et al . 1978).