Three important points must be emphasized in summary. First is the idea that a cellular microcompartment need not be limited by a semi-permeable membrane. We recognize microcompartments in multi-enzyme complexes where substrates are covalently transported from subunit to subunit. An example of this is the lipoic acid moiety of the pyruvate dehydrogenase complex. However, to act as a kinetic microcompartment, covalent transfer is not an obligatory requirement. Proximity effects may be sufficient for substantial rate enhancement. Our data clearly show that the kinetics of ADP translocation are influenced by the site of ADP formation. We contend that this represents a newly recognized and important form of cellular microcompartmentation. The second point is that we do not want our results misinterpreted as an overextension of the known data concerning tissue respiration. We believe that the primary parameter controlling heart mitochondrial oxygen consumption is the availability of ADP at the adenine nucleotide translocase. Our data show, however, that this is not a simple process. Secondary control is exerted by the localization of ADP formation, i.e. microcompartmentation. As a result of the kinetic data (Table 3), we conclude that the forward rate of mitochondrial creatine kinase is the preferential reaction controlling ADP delivery to the translocase. We are left, nonetheless, with questions concerning the secondary regulation of this enzyme in vivo by substrate (ATP and creatine) and inhibition by product (phosphocreatine). The nature of this control awaits further experimental data. Finally, the results are consistent with the creatine kinase energy transport hypothesis. Overall, the rate of tissue oxygen consumption reflects the metabolic activity of the organ, determined by the rate of ATP utilization (see right side of Figure 1). This results in the cytoplasmic production of ADP. In heart, this is coupled via the bound cytoplasmic isozymes of creatine kinase to the local rephosphorylation of ADP to ATP and the simultaneous production of creatine.(ABSTRACT TRUNCATED AT 400 WORDS)