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Tracy T. Nguyen, Kenichi Iwadate, Shimin Li, Joseph A. Bonanno; Distribution of Lactate Dehydrogenase Isoforms in Bovine Corneal Epithelium and Endothelium. Invest. Ophthalmol. Vis. Sci. 2011;52(14):274.
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© ARVO (1962-2015); The Authors (2016-present)
Lactate dehydrogenase (LDH) is an enzyme that interconverts the metabolism of pyruvate and lactate. LDH is a tetramer protein that is composed of two kinds of subunits, LDH-A (muscle type) and LDH-B (heart type). The composition of the two subunits determines the LDH presence in five isoforms that exhibit differential function, which control cell metabolic status. A higher ratio of LDH-B, such as in heart, indicates that the cell is optimized to convert lactate to pyruvate whereas a higher ratio of LDH-A subunits, such as in muscle, favors conversion of pyruvate to lactate. Determining the predominant LDH isoform in the cornea, specifically in epithelium and endothelium, will help us elucidate if these tissues are glycolytic or oxidative. In the present study, we examined the relative concentration of LDH-A and LDH-B subunits in bovine corneal epithelium and endothelium.
Fresh bovine eyes were dissected to collect corneal epithelium and endothelium tissues. Total RNA was isolated from these tissues along with beef muscle, liver and heart tissues which were used as positive controls. LDH-A and LDH-B expression were detected by RT-PCR and Western blotting. The relative amount of expression was quantified by real-time PCR.
LDH-A and LDH-B mRNA and protein were both detectable in all tissues. As expected, real-time PCR results revealed that in the heart, LDH-B is expressed at about twice as much as LDH-A, and vice-versa in muscle. In the liver, the ratio of LDH-A and LDH-B is about 1:1. In the corneal epithelium and endothelium, the LDH-A level is significantly higher than LHD-B, with a 5:1 ratio in the epithelium and 20:1 ratio in the endothelium.
Our results demonstrate that the muscle subunit (LDH-A) is dominant in both the corneal epithelium and endothelium, indicating that LDH favors the conversion of pyruvate to lactate. This is consistent with the highly glycolytic nature of corneal epithelial cells, but surprising for the endothelium, which is rich in mitochondria and has a relatively high oxidative potential. This finding suggests that corneal endothelial cells will prefer to metabolize glucose over the copious amounts of lactate diffusing from the epithelium/stroma to the anterior chamber. Further metabolic studies are needed to test this hypothesis.
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