Abstract
The turnover of cerebroside sulfate (sulfatide) was followed in both microsomal and myelin fractions of developing and adult rat brains after an intracerebral injection of Na(2)(35)SO(4). In the adult rats, the specific radioactivity of sulfatide of the microsomal fraction reached a maximum 12 hr after the injection, and after 3 days it was reduced to less than 30% of the maximum. In contrast, the specific radioactivity of the myelin sulfatide did not reach a peak until 3 days after the injection and remained essentially at the same level for as long as 6 months. In the case of 17-day-old rats, the specific radioactivity of myelin sulfatide reached a maximum level around 12 hr after the injection and then appeared to decline. The decline was most marked 2-6 days after the injection, suggesting an apparently rapid turnover of myelin sulfatide. When a correction was made for deposition of newly formed sulfatide, the results indicated that the turnover of myelin in the developing animals was also relatively slow. In vitro experiments with purified myelin and 3'-phosphoadenosine-5'-[(35)S]phosphosulfate showed that myelin does not catalyze the galactocerebroside sulfotransferase reaction. This enzyme was found mainly in the microsomal fraction. In vivo studies suggested that a transfer of sulfatide molecules from the endoplasmic reticulum to myelin might occur. In order to obtain direct evidence for such a transfer, rat brain slices after pulse labeling with Na(2)(35)SO(4) were washed free of the isotope and reincubated with nonlabeled Na(2)SO(4). The specific radioactivity of the microsomal sulfatide declined, with a concomitant rise in the specific radioactivity of the myelin sulfatide. These observations are therefore consistent with the postulate that myelin sulfatide is probably synthesized in the endoplasmic reticulum.
Highlights
Myelin sulfatide did not reach a peak until 3 days after the injection and remained essentially at the same level for as long as
T h e incubationswere noct ontinued for more than 5 hr because in other experiments it was noticed that therate of formation of sulfatide inthesubcellular fractions of the slices beganfalling off after 5 hr
Inthelatter three cases, significant radioactivity and, presumably, the precursor were present in the brain even after 7-20 days, and measurable radioactivity could still be found at longer time periods up to 3 months. It appears from these results that metabolism of sodium sulfate and itseventual elimination from rat brain is afairly rapid process compared withphospholipidprecursors described
Summary
Myelin sulfatide did not reach a peak until 3 days after the injection and remained essentially at the same level for as long as. Metabolicstudies of myelin using variousradioactive precursors of lipids and proteins have been interpreted in thepasttoindicatethat myelin intheadultanimal is rather an inert structure [1] In those studies the precursor was injected intotheanimalseitherintravenously or intraperitoneally.recenst tudies in which there was intracerebral or intraventricular administration of the precursors of phospholipids have given results indicating that a substantialpart of thephospholipid molecules in theadult myelin is in a fairly rapidequilibriumwith other components of brain tissue ( 2 4 ). It appears that the route of administration of the isotope andthe biological property of the precursor used may account to some extent for thecontradictoryresults. It has been suggested that myelin does not contain the Abbreviations: TLC, thin-layecrhromatographyP;APS3,”pbosphoadenosine-5’-phosphosulfate
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