The gel state of the vitreous and ascorbate-dependent oxygen consumption: relationship to the etiology of nuclear cataracts

Abstract

Objective: To investigate the rate and mechanism of oxygen consumption by the vitreous.

Methods: Oxygen consumption was measured with a microrespirometer. Vitreous ascorbate was measured spectrophotometrically and by gas chromatography-mass spectrometry. Vitreous degeneration was related to the rate of oxygen consumption and ascorbate concentration in samples obtained during vitrectomy.

Results: Prolonged exposure to oxygen or treatment with ascorbate oxidase eliminated oxygen consumption by the vitreous. Adding ascorbate restored oxygen consumption. Oxygen consumption persisted after boiling or treating the vitreous with the chelating agents EDTA and deferoxamine. In patients undergoing retinal surgery, liquefaction of the vitreous and previous vitrectomy were associated with decreased ascorbate concentration and lower oxygen consumption.

Conclusions: Ascorbate in the vitreous decreases exposure of the lens to oxygen. The catalyst for this reaction is not known, although free iron may contribute. The gel state of the vitreous preserves ascorbate levels, thereby sustaining oxygen consumption. Vitrectomy or advanced vitreous degeneration may increase exposure of the lens to oxygen, promoting the progression of nuclear cataracts.

Clinical relevance: Determining how the eye is protected from nuclear cataracts should suggest treatments to reduce their incidence.

Figure 1

Measurement of oxygen consumption and verification of ascorbate assay. A, Representative graph showing the change in oxygen consumption with time in vitreous obtained from a donor eye. B, Reconstructed ion chromatogram showing the results of gas chromatography–mass spectrometry analysis using selected ion monitoring of a sample of uniformly carbon 13–labeled ascorbic acid (¹³C₆-ascorbic acid) (upper trace) and ascorbate in human vitreous (lower trace). m:z indicates mass to charge ratio.

Figure 2

Results of oxygen consumption studies. A, Oxygen consumption in untreated donor vitreous or vitreous exposed to 5% oxygen for 48 hours. B, Changes in ascorbate concentration during exposure of donor vitreous to 5% oxygen. The line is the best fit of the data to a first-order reaction. C, Oxygen consumption by untreated vitreous or vitreous brought to a concentration of 10mM with EDTA. D, Oxygen consumption by human vitreous (4.9 µL/mL/h) and vitreous supplemented with deferoxamine mesylate (DFO) to a final concentration of 100µM DFO (2.9 µL/mL/h).

Figure 3

Mean rate of oxygen consumption (A) and mean ascorbate concentration (B) in vitreous samples obtained at the time of surgery. In patients who were undergoing their first vitrectomy, the gel status of the vitreous was scored on a 5-point scale. Vitreous that received a score of 3 or less had firmer gel; greater than 3, more liquid. *P>.05. †P<.001. ‡P<.01.

Figure 4

Scatterplots of oxygen consumption and ascorbate concentration. A, Oxygen consumption and ascorbate concentration relative to the vitreous liquefaction score of samples obtained from patients with no previous vitrectomy. Lower liquefaction score indicates a firmer gel; higher numbers are more liquid. The best linear fit to the oxygen consumption data is shown by the dashed line and that to the ascorbate concentration by the solid line. B, Relationship between oxygen consumption and ascorbate concentration for the 48 samples in which both were measured.