Vitamin A in human eyes: amount, distribution, and composition

Area of Science:

• Ophthalmology
• Nutritional Science
• Biochemistry

Background:

• Vitamin A is essential for vision, playing a critical role in the visual cycle.
• The eye, particularly the retinal pigment epithelium-choroid (RPE-Ch), is known to store vitamin A, but its precise levels and forms in human postmortem eyes are not fully characterized.

Purpose of the Study:

• To quantify the amount, distribution, and composition of vitamin A in human postmortem eyes.
• To investigate the stability of vitamin A in ocular tissues after death and during storage.
• To assess the functional significance of stored vitamin A for rhodopsin regeneration.

Main Methods:

• High-pressure liquid chromatography (HPLC) was used to determine vitamin A concentration and composition.
• Rhodopsin regeneration assays were performed on retinal homogenates.
• Analysis of vitamin A esterification in intact RPE-Ch and isolated RPE cells using radiolabeled retinol.

Main Results:

• The RPE-Ch contains the highest concentration of vitamin A in non-liver human tissue, with no significant loss postmortem or during cold storage.
• Stored vitamin A in the RPE-Ch is predominantly esterified (98.3%) with stearate and palmitate, and is sufficient to regenerate rhodopsin.
• Retinal pigment epithelium cells esterify exogenous retinol, primarily retaining the all-trans configuration.

Conclusions:

• The human RPE-Ch is a major reservoir for vitamin A, crucial for maintaining visual function.
• Vitamin A stores in the eye are stable and readily available for the visual cycle.
• Understanding ocular vitamin A storage provides insights into visual physiology and potential therapeutic strategies.