Technology for portable measurement of blood lactate.

Abstract

929 GLUCOSE IS METABOLIZED by cells to produce energy. Glucose metabolism involves progressive oxidation and breakage of carbon bonds. The oxidation process causes C-H and C-C bonds to be stripped of electrons (oxidized), which are then used to build ATP. The initial steps in breakdown of glucose involve conversion of one six-carbon molecule of glucose to two three-carbon molecules of pyruvate. This process is known as glycolysis. Next, in the presence of oxygen, the carbon atoms in pyruvate are converted into three molecules of carbon dioxide in a process known as aerobic metabolism. When oxygen is available to serve as the final acceptor of electrons, then pyruvate (by way of a series of steps known as the Krebs cycle or tricarboxycylic acid cycle) is able to transfer electrons to (or reduce), the final acceptor, which is oxygen. When oxygen is totally reduced, then it becomes H2O or water. Meanwhile, the carbon bonds of pyruvate all become oxidized to CO2. Conversely, in the absence of oxygen, all the electron acceptors “downstream” from pyruvate are reduced and unable to offload electrons to mediators that will carry them towards oxygen. The carbon bonds are progressively oxidized in the Krebs cycle, and the electrons’ energy is drawn off in steps through a process known as oxidative phosphorylation. The process is analogous to water falling down a dam and turning turbines while falling, and at the same time the turbines transfer energy to generators that produce electric power. The Krebs cycle is the dam, and the oxidative phosphorylation is the generator. Anaerobic metabolism is a state of water being backed up downstream so that there is no flow of water across the dam. Without oxygen, the backup of reduced substances reaches pyruvate, which cannot transfer its electrons into any chemicals within the Krebs cycle. Pyruvate itself then becomes reduced to lactate and broken down no further. The metabolic process that begins with glucose and ends with lactate is known as anaerobic metabolism. Lactate does not usually accumulate when oxygen is available.2 Aerobic metabolism is preferable to anaerobic metabolism. More energy (defined as the number of ATP molecules generated per glucose molecule broken down) is derived from aerobic metabolism than from anaerobic metabolism. The combination of glucose ignition by way of glycolysis, Krebs cycle, and oxidative phosphorylation in aerobic metabolism generates 36 ATP molecules per glucose molecule compared with only 2 ATP molecules per glucose molecule that are generated in anaerobic metabolism by glycolysis alone. When exercise is continues past the point of adequate oxygen delivery (such as during excessive training beyond the ability of cardiac output to supply adequate blood), then glucose breakdown switches from aerobic to anaerobic metabolism. Lactic acid builds up, and the acid dissociates to lactate plus free hydrogen ions,