The regulation of metabolism may be from within the cell or outside. The metabolic flux can be regulated by non-equilibrium reactions. The intracellular regulatory strategies include allosteric enzymes, substrate cycles, enzyme interconversion cycles etc. the cyclic AMP and phosphoinositide systems are major mechanisms of signal transduction. Metabolism is also regulated by hydrophobic hormones which enter their target cells and are able to interact with intracellular receptor molecules. The movement of substrates into the cells compartments is also controlled by the energy charge. The charge depends on the amounts of ATP, ADP and AMP which are all relatively regulated.