Thyroid lysosomes: the stability of the lysosomal membrane

To determine the integrity of lysosomes during their isolation from rat thyroid glands and their subsequent incubation at 37 degrees C, the sedimentability of lysosomal acid phosphatase and thyroglobulin (amount of undisrupted lysosomes) and the latency of sedimentable acid phosphatase (permeability of undisrupted lysosomes) were measured concomitantly. The following results were obtained: (a) During isolation of lysosomes in 0.25 M sucrose medium, mild homogenization of thyroid tissue or cholesterol addition did not modify the amount of undisrupted lysosomes but reduced their permeability. Homogenization in 0.5 M sucrose decreased both the amount and the permeability of undisrupted lysosomes. It also reduced their content of recently iodinated thyroglobulin (Tg). Cholesterol addition had no effect in 0.5 M sucrose medium. (b) During incubations at 37 degrees C of lysosomes, the amount of undisrupted lysosomes decreased progressively while their permeability increased. According to the incubation pH, the permeability of lysosomes prepared in 0.25 M sucrose was either more (pH 8) or less (pH 6) extensively increased than that of lysosomes prepared in 0.5 M sucrose. From these results, we concluded: (a) that isolation and incubation of the thyroid lysosomal fraction induce increased permeability of lysosomes prior to their complete disruption: (b) that recently formed lysosomes (high content of recently iodinated Tg) and aged lysosomes (low content of recently iodinated Tg) differ significantly. Recently formed lysosomes are more permeable, are stabilized by cholesterol and are more extensively disrupted in 0.5 M sucrose medium. During incubations, the permeabilities of these two classes of lysosomes are also differently affected by external pH.

Intralysosomal hydrolysis of thyroglobulin. I. Modulation by lysosomal membrane permeability and exogenous factors

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.

Intralysosomal hydrolysis of thyroglobulin. II. Different fates of poorly and fully iodinated Tg and specific activation by TSH of the degradation of fully iodinated Tg

Degradations of recently (1 h) iodinated thyroglobulin (Tg) and of Tg iodinated for 48 h (fully iodinated molecule) inside thyroid phagolysosomes have been measured in vitro, at different pH, in basal (lysosomes from normal rats) and stimulated (lysosomes from TSH-treated rats) conditions. In basal conditions, recently iodinated Tg was shown to be preferentially degraded at pH 5 but not at pH 8. Mercaptoethanol (ME) was found to preferentially stimulate the degradation of 48 h-iodinated Tg at pH 5 but not at pH 8. In stimulated conditions and in the presence of ME, no preferential hydrolysis of recently iodinated Tg was observed, whatever the pH. Since the degradation of Tg and its stimulation by ME are known to depend on the mean iodine content of the molecule, it was concluded that: 1) the recently iodinated Tg population contains both poorly and fully iodinated molecules, the degradations of which are faster or similar to that of 48 h-iodinated Tg, respectively, 2) among thyroid phagolysosomes, only those which exhibit a pH-dependent proteolytic activity contain poorly iodinated molecules while others, which are functional at either pH, contain solely fully iodinated molecules (1 and 48 h labelled), 3) reduction of the disulphide bridges of fully iodinated Tg, mediated by ME and probably a lysosomal transhydrogenase, is a prerequisite for its optimal degradation by lysosomal proteinases, 4) in addition, TSH, which preferentially stimulates the degradation of fully iodinated Tg, was concluded to interfere directly upon the intralysosomal hydrolytic process, independently of its effect upon Tg endocytosis. This effect of TSH, similar and additive to that of ME suggests that the hormone might activate a lysosomal transhydrogenase.

Iodinated particles in the rat thyroid. II. Lysosomal characteristics

In a previous study (Simon et al. 1971) a procedure for the preparation and separation of iodinated particles was described in the rat. The present paper deals with further investigations on the nature of these particles. Acid phosphatase and iodine are conjointly sedimentable and display a latency that is unmasked on dilution in a hypo-osmotic medium and under acidification to pH 5.0. These properties together with the sensitivity to Triton X-100 are best accounted for by assuming that iodinated particles of the thyroid gland are lysosomes. Part of the particulate iodine is soluble in n-butanol (BEI fraction). The existence of this BEI fraction demonstrates that hydrolysis of thyroglobulin occurs within the particles which thus exhibit an acid protease activity. Both the sedimentable iodine pool and acid phosphatase are increased under TSH stimulation and decreased after thyroxine treatment. In addition, the general activity of the iodinated particles is dependent on the daily iodine intake as shown by the variation of their iodine pool, acid phosphatase activity and BEI fraction with the iodine diet. It is concluded that iodinated particles of the thyroid gland are secondary lysosomes which participate in iodine secretion under TSH control. By in vitro treatment with destabilizing media or after in vivo treatment with thyroxine, iodinated particles exhibit a parallel loss of iodine and acid phosphatase. After a short-term TSH treatment in vivo, their iodine pool is more increased than their acid phosphatase activity. It is concluded that, at least in the normal rat thyroid, iodinated particles are essentially secondary lysosomes; true colloid droplets actually accumulate only after sufficient TSH stimulation.

Iodinated particles in the rat thyroid. I. A rapid separation method

In a first step, the quantitative preparation of the whole population of iodinated particles was reinvestigated in thyroids obtained from rats maintained in an isotopic equilibrium with 125I. The maximum yield in the iodinated particles was obtained for a 0.15 m sucrose solution in 0.1 m Tris-HCl buffer pH 7. This yield was 3.5 % and 1.7 % of the total iodine content of the thyroid for rats adapted to receive 50 (group 50) and 5 μg (group 5) of iodine daily respectively. The iodinated particles of group 50 were found to be more sensitive to pH variations than those of group 5.

In a second step, a rapid method was developed to separate the whole population of iodinated particles into physiological populations (tentatively colloid droplets and secondary lysosomes). By centrifuging onto a discontinuous Ficoll gradient, three distinct iodine peaks were regularly obtained for rats of group 5 and 50 and also for other rats receiving a diet rich in iodine. For group 50, the iodine pools (127I) as measured by isotopic equilibrium were found to represent about 16% (peak 1), 25 % (peak 2) and 59 % (peak 3) of the total particulate iodine content of the gland.

The possible nature of these three populations of iodinated particles is discussed.