Granular and extra-granular forms of immunoreactive calcitonin in normal rat "C" cells

The intracellular localization of the intermediate calcitonin-related peptides produced during calcitonin maturation (CT, 3.4 Kd) from the polyprotein precursor (15 Kd) has been studied by means of subcellular fractionation of normal adult rat thyroid using a continuous hypertonic Percoll gradient. A fraction B (Bottom), containing the secretory granules, was separated from a fraction T (Top) constituted of rough and smooth membranes devoid of granules. Immunoreactive calcitonin extracted from both fractions was heterogeneous when analysed by gel electrophoresis followed by radioimmunoassay. In fraction T, the immunoreactive molecules assayed were associated with the membranes. Their apparent molecular weights (lower than 18 Kd) were compatible with the intermediates predicted by the aminoacid sequence of the precursor. However the maturation was not complete, as indicated by the absence of the monomer. The smallest molecular weight detected (4-4.5 Kd) was compatible with a calcitonin-katacalcin uncleaved molecule. These data strongly suggest the occurrence of a cleavage separating the N-terminal part of the molecule during the pre-granular step of calcitonin secretory pathway. In the granular fraction, the monomer constituted the granular matrix, representing 65% of the total immunoreactivity. The remainder was associated with the granules membranes, consisting of the 4-4.5 Kd precursor and some 43 Kd material which has already been demonstrated by others to be covalent polymers of calcitonin [17]. It is thus concluded that a second cleavage could take place in the granules where the major immunoreactive form is the processed calcitonin, either as free monomer or as covalent polymers associated with the membrane of the granules.