A majority of ATP in the brain is formed in the mitochondria through oxidative phosphorylation of ADP with the F1F0-ATP (ATPase) enzyme. This ATP production rate plays central roles in brain bioenergetics, function and neurodegeneration. In vivo 31 P magnetic resonance spectroscopy combined with magnetization transfer (MT) is the sole approach able to noninvasively determine this ATP metabolic rate via measuring the forward ATPase reaction flux (F f,ATPase). However, previous studies indicate lack of quantitative agreement between F f,ATPase and oxidative metabolic rate in heart and liver. In contrast, recent work has shown that F f,ATPase might reflect oxidative phosphorylation rate in resting human brains. We have conducted an animal study, using rats under varied brain activity levels from light anesthesia to isoelectric state, to examine whether the in vivo 31 P MT approach is suitable for measuring the oxidative phosphorylation rate and its change associated with varied brain activity. Our results conclude that the measured F f,ATPase reflects the oxidative phosphorylation rate in resting rat brains, that this flux is tightly correlated to the change of energy demand under varied brain activity levels, and that a significant amount of ATP energy is required for ''housekeeping'' under the isoelectric state. These findings reveal distinguishable characteristics of ATP metabolism between the brain and heart, and they highlight the importance of in vivo 31 P MT approach to potentially provide a unique and powerful neuroimaging modality for noninvasively studying the cerebral ATP metabolic network and its central role in bioenergetics associated with brain function, activation, and diseases.A denosine triphosphate (ATP), a high-energy phosphate (HEP) compound, is the universal energy currency in living cells for supporting the energy needs of various cellular activities and functions. In the brain, a majority of ATP is formed in the mitochondria through oxidative phosphorylation of adenosine diphosphate (ADP) catalyzed by the enzyme of ATP synthase (ATPase) (1). A large portion of ATP energy is used in cytosol to pump sodium and potassium across the cellular membrane for maintaining transmembrane ion gradients and to support neurotransmitters cycling and, thus, sustaining electrophysiological activity and cell signaling in the brain. The ATP metabolism regulating both ATP production and utilization plays a fundamental role in cerebral bioenergetics, brain function, and neurodegenerative diseases (2-6).The brain ATP metabolism is mainly controlled by ATPase and creatine kinase (CK) reactions that are coupled together and constitute a complex chemical exchange system involving ATP, phosphocreatine (PCr), and intracellular inorganic phosphate (Pi) (i.e., a PCr^ATP^Pi chemical exchange system) (7-10). One vital function of this ATP metabolic network is to maintain a stable cellular ATP concentration by adjusting the reaction rates to ensure a continuous energy supply for sustaining electrophysiological activity and ...

ATP metabolism is controlled mainly by ATP synthase (ATP ase ) and creatine kinase (CK) reactions that regulate cerebral ATP production, transportation, and utilization. These coupled reactions constitute a chemical exchange metabolic network of PCr7ATP7Pi characterized by two forward and two reverse reaction fluxes, which can be studied noninvasively by in vivo 31 P MRS combined with magnetization transfer (MT). However, it is still debated whether current MT approaches can precisely determine all of these fluxes. We developed and tested a modified in vivo 31 P MT approach based on a multiple single-site saturation (MSS) technique to study the entire PCr7ATP7Pi network in human occipital lobe at 7T. Our results reveal that 1) the MSS MT approach can explicitly determine all four reaction fluxes with a minimal number of The primary functions of brain cells are excitation and conduction, which are reflected by constant electrophysiological activity in the brain. The cerebral bioenergetics that support sustained electrophysiological activity are ultimately driven by a variety of biochemical processes that maintain the normal function and structural integrity of the brain (1). Of these processes, the most fundamental for supporting various cellular activities is adenosine triphosphate (ATP) metabolism in living cells (2). The majority of ATP is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi) in the mitochondria through oxidative phosphorylation catalyzed by the ATP synthase (ATP ase ) enzyme, as illustrated by Fig. 1a (3,4). The highly demanding biochemical processes involving ATP production and utilization in the brain cause rapid chemical cycling among ATP, ADP, and Pi (see Fig. 1a). These processes are also accompanied by another important chemical reaction involving phosphocreatine (PCr) and creatine kinase (CK). PCr acts as an ATP reservoir and carrier, and transfers energy from the mitochondria to sites of ATP utilization in the cytosol through reversible CK reactions, ultimately maintaining a stable cellular ATP level (4,5). These two chemical exchange reactions (i.e., PCr7ATP and Pi7ATP) play central roles in regulating ATP metabolism and maintaining normal ATP functionality, both of which are crucial for cerebral bioenergetics and brain function in the healthy brain as well as in neurodegenerative diseases. Moreover, the ATP ase and CK reactions are tightly coupled together, leading to a complex three-31 P-spin chemical exchange kinetic network (i.e., PCr7ATP7Pi) as depicted in Fig. 1b. Thus, it is essential to develop a noninvasive, reliable technique that is capable of assessing the entire kinetic network of PCr7ATP7Pi and associated ATP metabolic fluxes in situ, particularly in the human brain.Measuring all kinetic parameters involved in the PCr7ATP7Pi network requires extensive information, including three steady-state phosphate metabolite concentrations (i.e., [ATP], [PCr], and [Pi]) and four pseudo-firstorder chemical reaction rate constants (forward and reverse rate constants fo...

Despite the essential role of the brain energy generated from ATP hydrolysis in supporting cortical neuronal activity and brain function, it is challenging to noninvasively image and directly quantify the energy expenditure in the human brain. In this study, we applied an advanced in vivo 31P MRS imaging approach to obtain regional cerebral metabolic rates of high-energy phosphate reactions catalyzed by ATPase (CMRATPase) and creatine kinase (CMRCK), and to determine CMRATPase and CMRCK in pure grey mater (GM) and white mater (WM), respectively. It was found that both ATPase and CK rates are three times higher in GM than WM; and CMRCK is seven times higher than CMRATPase in GM and WM. Among the total brain ATP consumption in the human cortical GM and WM, 77% of them are used by GM in which approximately 96% is by neurons. A single cortical neuron utilizes approximately 4.7 billion ATPs per second in a resting human brain. This study demonstrates the unique utility of in vivo 31P MRS imaging modality for direct imaging of brain energy generated from ATP hydrolysis, and provides new insights into the human brain energetics and its role in supporting neuronal activity and brain function.