In the rat heart, monoamine oxidase (MAO)-B activity was
shown to predominate in 2- to 3-week-old animals, whereas
MAO-A activity was reported to be very low in newborn rats
and to increase considerably with age until it predominates.
These results are in contrast with those found in the mouse
heart, where an age-dependent increase in MAO-B activity
with no changes in 5-hydroxytryptamine deaminating activity
was found to occur. There is evidence that the adult values of
MAO activity are reached early in development in rat kidney
and liver. In the rat lung the adult values of MAO-A activity
are reached by day 40, whereas MAO-B activity is still increasing
by day 80. Important differences have been reported in the
developmental pattern of the two forms of MAO in the rat and
mouse brain, with a decrease in the MAO-A/MAO-B ratio
during postnatal development. In the human brain, the ontogenetic
development of MAO-A and MAO-B appears to parallel
that observed in the rodent brain. It is worth noting that
most of the available data have to be considered with reservation
owing to many methodological problems. Further studies
are clearly needed to get reliable information on the ontogenesis
of MAO in mammalian tissues.
Eye MAO-A, MAO-B, semicarbazide-sensitive amine oxidase (SSAO) and aldehyde reductase (AR) activities were measured in young and old rats. When enzyme activity is expressed as nmol (mg protein)-1 min-1, a significant decrease (18-23%) of SSAO activity in the eye of old rats was found, whereas there was no significant difference in MAO-A and MAO-B activities. A significant increase of AR activity with D-xylose (67%), DL-glyceraldehyde (64%), D-glucuronate (43%) and D-glucose (21%) was found in the eye of old rats. These results suggest that changes in the activities of the amine metabolizing enzymes of rat eye with age might have consequences for their function in senescence; particularly, the increase of AR activity might be involved in cataract formation.
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