Distribution of molecule numbers per secretion granule. A study of crystals in glucagon secreting cells

The occurrence of a crystalline component in secretion granules of teleost A-cells (islets of Langerhans) has been used to derive, in Xi-phophorus helleri, the size distribution of secretion granules in terms of (glucagon) molecule numbers. The distribution is asymmetric with the arithmetical mean = 7.1 x 10(5), the median Q0.5 = 4.7 x 10(5), and the range from 8.5 x 10(4) to 4.5 x 10(6) molecules, as determined in a sample of 203 crystals from 9 specimens. This result is based on the following assumptions: The crystals are rhombic dodecahedra with a greater than 4.2 nm and space group P213, thus much resembling cubic porcine glucagon; they are teleost glucagon and contain the whole glucagon content of the granule. If the latter assumptions are rejected, the above molecule numbers become minimum numbers of protein molecules. A change to other space groups also compatible with the diffraction patterns observed would only double the above values. The values are compared with corresponding ones from other species and also from other types of endocrine cells and errors are discussed.