The neuraminidases of rat eyes

A micromethod for the determination of iron and total iron-binding capacity in intraocular fluids and plasma using electrothermal atomic absorption spectroscopy

A new micromethod (requiring only 30 microliters of sample) for the determination of iron (Fe) concentration and total iron-binding capacity (TIBC) in intraocular fluids (IOFs; aqueous and vitreous humors) and plasma has been developed using electrothermal atomic absorption spectrophotometry. Values for Fe concentration in the IOFs were similar between species and were generally higher in the vitreous (0.031-0.060 mg/liter) than aqueous (0.014-0.038 mg/liter) humors obtained from the same eye. Plasma Fe concentration and TIBC fell within established limits for all species studied. The iron binding capacity of the aqueous and vitreous humors from the same species is similar. While the TIBC of the aqueous humor is saturated to the same extent as plasma, vitreous TIBC is more highly saturated in all species. Using this method considerably less species variation in IOF Fe concentration was found as compared to previous studies. Some possible methodological reasons for this difference are discussed.

Properties of endogenous, membrane-associated sialidase activity (N-acetylneuraminidase) of the goldfish visual system

The endogenous sialidase (N-acetylneuraminidase) activity of membranes prepared from goldfish retina and optic tectum displays characteristics similar to those reported for neural plasma membrane sialidases of other organisms. Endogenous membrane sialidase activity was found to be optimal at ph 4.0, and maximal release was obtained at 37-50 degrees C, above which temperature thermal instability of the preparations was observed. Optic nerve crush, which results in regeneration of retinal ganglion cell axons, did not result in significant changes in measured endogenous membrane sialidase activity in either the retina or the optic tectum. Enzymatic hydrolysis of membrane sialoglycolipid (ganglioside) accounted for about 70% of the total sialic acid released. Ganglioside GM1 accumulated as the major lipid product in both retina and tectum, indicating that the inner sialosylgalactosyl linkage in the ganglio oligosaccharide series was resistant to hydrolysis by the endogenous enzyme.

Neuraminidase in calf retinal outer segment membranes

An enzyme catalyzing the hydrolysis of sialic acid (N-acetylneuraminic acid: NeuNAc)-containing glycoconjugates has been found in bovine retinal rod outer segment (ROS) membranes. The enzymatic activity is optimal at pH 4.0 and is stimulated by 0.15% Triton X-100. Total activity was determined by the release of NeuNac from endogenous and exogenous substrates (GD1a). The ROS enzyme preferentially hydrolyses the ROS gangliosides, possibly because they are more accessible than the glycoproteins as substrates for the neuraminidase. Release of NeuNAc from gangliosides leads to important changes in the ganglioside patterns; whereas the amounts of GM1 increased throughout the incubation, the levels of polysialogangliosides GT1b and GD3 diminished owing to their rapid hydrolysis. The finding that gangliosides are hydrolysed more extensively than glycoproteins suggests that endogenous ROS gangliosides may be the principal source of metabolically available sialic acid in ROS. It was also observed that the activity of ROS neuraminidase is not affected by illumination of the membranes.

Enzyme activities and protein concentration in the intraocular fluids of ten mammals

An attempt was made to establish normal values for the total protein concentrations and the enzyme activities of LDH, MDH and PGI in the intraocular fluids of rats, guinea pigs, rabbits, cats, dogs, sheep, cattle, pigs, horses and humans. Remarkably little species differences were noted in 9 of the 10 mammals with vitreal enzyme activities falling into a narrow range between 8.4 U/l (PGI, horse) and 92.4 U/l (MDH, guinea pig). All species obeyed the sequence aqueous less than vitreous less than serum with exception of the rat, where vitreous activities surpassed serum at least two-fold. The very low enzyme activities in the aqueous and vitreous humors demand special precautions against contaminations which can either derive from post-mortem storage or inadequate sampling procedures. Conversely elevated enzyme activities could be used as probes for pathological processes or as a check for a clean preparation.