Filter-grown Madin-Darby canine kidney (MDCK) cells labelled for 24 h with [35S]sulphate were found to secrete macromolecules [35S]sulphated on their carbohydrate moieties predominantly into the basolateral medium, whereas the tyrosine-[35S]sulphated proteins synthesized were predominantly secreted into the apical medium. In contrast with the predominant apical secretin of tyrosine-[35S]sulphated proteins, the free tyrosine O-[35S]sulphate (Tyr[35S]) was released mostly into the basolateral medium. A time-lapse study using prelabelled MDCK cells incubated in fresh medium revealed that, during the 48 h time course monitored, the release of tyrosine-[35S]sulphated proteins into the apical medium was faster and quantitatively greater than that into the basolateral medium. During the same time there was a concomitant release, predominantly into the basolateral medium, of the free Tyr[35S] derived from the degradation of tyrosine-[35S]sulphated proteins. An endocytotic degradation experiment was performed to demonstrate the endocytosis of tyrosine-sulphated proteins and their degradation to generate free TyrS. It was found that free Tyr[35S] was generated and released when an apically secreted (or basolaterally secreted) tyrosine-[35S]sulphated protein preparation was added to the apical medium (or the basolateral medium) of unlabelled filter-grown MDCK cells. In both cases, the free Tyr[35S] generated was predominantly released into the basolateral medium similar to the results obtained in the time-lapse study.

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