Diurnal variation in human tear enzymes

A fluorometric technique for measuring the activities of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) in human tear fluid is described. Measurement of separate LDH and MDH activities during the day resulted in fluctuating values with no definite pattern. However, a consistent diurnal pattern emerged when the tear LDH/MDH ratio was measured for individual tear sample. The tear LDH/MDH ratio was highest upon waking, declining soon after to a stable level for the remainder of the day. Evidence for a corneal epithelial origin of tear LDH and MDH is discussed, as well as the ability of the technique to detect differences in the metabolic status of the epithelium between the open-eye and closed-eye environments. Possible causes of the elevation of the tear LDH/MDH ratio following lid closure are unbinding of intracellular M-type LDH within epithelial cells and increased cell membrane permeability, both processes occurring in response to reduced cellular energy content under the more hypoxic conditions of lid closure. Although the nature of the present experiment did not allow any conclusive statement, de novo LDH synthesis may also be contributing to the increased tear LDH/MDH ratio. Deliberate corneal epithelial surface disruption was found to have no quantifiable effect on the tear LDH/MDH ratio.