Intralysosomal hydrolysis of thyroglobulin

Abstract. Intralysosomal hydrolysis of endogenous (125I in vivo labelled) thyroglobulin (Tg) inside thyroid phagolysosomes and its modulation by exogenous factors (external pH, activators and inhibitors of lysosomal proteinases) were studied during in vitro incubations. Tg degradation was followed as the time-dependent increase in TCA-soluble radioiodine (free iodoaminoacids). The results demonstrate that this hydrolysis is only partially pH-dependent, one component being abolished by pH 8 and a cathepsin B inhibitor (zinc) whereas the residual activity was pH-independent and insensitive to zinc. It would seem therefore that the total thyroid phagolysosome population contains lysosomes permeable to both incubation buffer and zinc and lysosomes impermeable to these compounds. Both classes are operational (degrade Tg) at acidic or neutral pHs, while only impermeable organelles function when incubated at pH 8. These impermeable lysosomes have an acidic internal pH (about pH 5) and resist alkalinization very efficiently due to the high impermeability of their membranes. Additional experiments are in progress to determine the physiological significance of these two classes of lysosomes which might be related to a functional maturation of thyroid phagolysosomes.