The lysosomal enzymes of the iris-ciliary body are retained in their organelles and exhibit increased activities during acute uveal inflammation

Phase I and phase II ocular metabolic activities and the role of metabolism in ophthalmic prodrug and codrug design and delivery

While the mammalian eye is seldom considered an organ of drug metabolism, the capacity for biotransformation is present. Compared to the liver, the metabolic capabilities of the eye are minuscule; however, phase I and phase II metabolic activities have been detected in various ocular structures. The careful consideration of ocular tissue metabolic processes within the eye has important implications for controlling the detoxification of therapeutic agents and for providing the potential for site-specific bio-activation of certain drug molecules, thus enabling significant improvements in drug efficacy and the minimization of side-effect from either local or systemic drug delivery to the eye. Knowledge of these processes is important to prodrug and codrug development and to researchers involved in the design, delivery and metabolism of ophthalmic drugs. This present article reviews the progress in ocular prodrug and codrug design and delivery in light of ocular metabolic activities.

Identification of a ubiquitin-like protein in the mammalian vitreous humor

An 8 kDa ubiquitin-like peptide (ULP) was isolated by high performance liquid chromatography from the rabbit vitreous humor, and the N-terminal amino acid sequence of this peptide showed complete homology with ubiquitin. Western blot revealed the presence of free ULP in both the iris-ciliary (IC) complex and the aqueous humor extracts. In the IC complex, fluorescence and immunoelectron microscopy detected high concentrations of ULP in the posterior epithelial cells, suggesting this tissue as a possible source of ULP in the ocular fluids. Significantly, this is the first time that the presence of free ULP has been reported in mammalian extracellular fluids. Furthermore, we recently demonstrated that the 8 kDa fraction of vitreous humor containing ULP is a potent inhibitor of protein synthesis [Banerjee et al. (1992): J Cell Biochem 49:66-73]. These findings taken together suggest a novel biological role for ULP in the control of lens cell growth.

The shape and distribution of lysosomes and endocytosis in the ciliary epithelial cells of rats

The shape and distribution of lysosomes in the ciliary epithelium of rat eyes were examined by electron microscopy combined with acid phosphatase (ACPase) cytochemistry and three-dimensional observation of 2 microns-thick sections. ACPase activity was cytochemically localized in lysosomes and trans Golgi cisternae in the non-pigmented epithelial (NPE) and pigmented epithelial (PE) cells. In NPE cells, it was shown three-dimensionally, that most lysosomes had an elongate form, up to 5 microns in length, and a diameter of 70-100 nm. These elongate lysosomes (nematolysosomes) were predominantly located in the basal region of the cells. In contrast, PE cells had spherical lysosomes distributed at random throughout the cytoplasm. However, no nematolysosomes were seen in the PE cells. When the isolated ciliary processes were incubated in a medium containing horseradish peroxidase (HRP), HRP was incorporated into the nematolysosome-like structures by pinocytosis from the basal surface of the NPE cells. These findings suggest that nematolysosomes are associated with the pinocytotic activity of NPE cells. The pinocytosis-nematolysosomal route may be involved in the uptake and degradation of macromolecules from the aqueous humor in the posterior chamber.

Histochemical survey of the anterior segment of the normal human foetal and adult eye

The distributions of the lysosomal enzymes [acid phosphatase (AP), N-acetyl-beta-D-glucosaminidase (NAG), beta-glucuronidase (beta-Gluc), beta-galactosidase (beta-Gal), dipeptidylpeptidase II (DPP II)] and of the membrane-bound proteases [aminopeptidase M (APM), aminopeptidase A (APA), gamma-glutamyltransferase (GGT), dipeptidylpeptidase IV (DPP IV)] were investigated in the normal human adult and foetal anterior segment by histochemical methods. The distribution of these hydrolases varied between ocular tissues. The most active enzymes in the adult corneal epithelium and endothelium were AP, beta-Gluc, NAG, beta-Gal and GGT; in the keratocytes, APM, APA, beta-Gluc and GGT predominated. The adult trabecular meshwork cells were stained by AP, beta-Gluc, NAG, APM, GGT, DPP II and DPP IV. The enzymes AP, beta-Gluc, APM and APA, however, displayed greater activity in the endothelium of Schlemm's canal. The adult ciliary epithelium stained strongly for all lysosomal hydrolases; GGT was the most active protease here. Differences in enzyme activity were noted in some tissues when foetal and adult anterior segments were compared. There appeared to be a decrease in the activity of some enzymes with age and post-mortem delay greater than 24 h. The function(s) of each enzyme and their possible roles in the respective tissues are discussed.