The immunology of insulin

Sequence of a New World primate insulin having low biological potency and immunoreactivity

The organization of the insulin gene of the owl or night monkey (Aotus trivirgatus), a New World primate, is similar to that of the human gene. The sequences of these two genes and flanking regions possess 84.3% homology. An unusual feature of the owl monkey gene is the partial duplication and insertion of a portion of the A-chain coding sequence into the 3' untranslated region. The insulin gene of this primate also lacks a region of tandem repeats that is present in the 5' flanking region of the human and chimpanzee genes. Owl monkey preproinsulin has 85.5% identity with the human insulin precursor and is the most divergent of the primate insulins/preproinsulins yet described. The differences between owl monkey and human preproinsulin include three substitutions in the signal peptide, two in the B chain, seven in the C peptide, and three in the A chain. One of these replacements is the conservative substitution of valine for isoleucine at position A2, an invariant site in all other vertebrate insulins and insulin-like growth factors. The substitutions in owl monkey insulin at B9, B27, A2, A4, and A17 alter its structure so that it has only 20% of the receptor-binding activity and 1% of the affinity with guinea pig anti-porcine insulin antibodies as compared to human insulin.

Identification of human glandular kallikrein in the beta cell of the pancreas

To determine the cellular localization of glandular kallikrein in the human pancreas, immunohistochemical studies were performed with a monospecific antibody against the antigenically identical urinary kallikrein (urokallikrein). The localization of glandular pancreatic kallikrein to the beta cells of the islets was the same as that of insulin in normal human pancreas and in two islet-cell tumors. When beta cells were lacking in islet-cell tumors or in the pancreas of a patient with juvenile-onset diabetes, kallikrein antigen was not detectable. Anti-urokallikrein absorbed with purified urinary or pancreatic kallikrein no longer identified a pancreatic antigen, whereas absorption with insulin had no effect. The beta-cell localization of human pancreatic kallikrein, an endopeptidase that, in concert with carboxypeptidase B, converts bovine proinsulin to a polypeptide with the electrophoretic mobility of insulin, suggests that pancreatic kallikrein may be involved in the physiologic activation of proinsulin.

Recognition of self and nonself, the crucial role of phagocytosis and lysosomal destruction of antigen

Primitive animals possess the ability to take up foreign matter and degrade it intracellularly. It is unlikely that during the evolution of the immune apparatus this mode of dealing with the foreign material would be bypassed. It is suggested that phagocytosis and intracellular digestion of the antigen is the primary mode of defense, and that failing complete digestion of the antigen an antibody response occurs. Phagocytosis of the antigen is known to be a necessary step in the process of antibody formation. Following the uptake by macrophages, the antigen is concentrated within the lysosome and subjected to the action of hydrolytic enzymes. Lysosomes are also the sites of catabolism of body constituents, and are believed to contain enzymes necessary for complete digestion of self constituents. If the lysosomal enzymes can completely degrade the antigen, the latter's possible harmful effects would be destroyed. The organism then does not need to waste energy in the synthesis of specific antibody, the function of which now is redundant. If the lysosomal enzymes cannot hydrolyze the antigen completely, the hydrolysis-resistant fragment can be transferred to lymphocytes, where it can induce antibody synthesis. Thus, the ability of the host hydrolytic enzymes to degrade a substance completely constitutes the recognition of self and the inability to do so amounts to the recognition of nonself.

Human insulin: facile synthesis by modification of porcine insulin

Human insulin differs from porcine insulin by a single amino acid- the carboxyl terminal residue of the B chain. By means of chemical and enzymatic treatment, it is possible to remove quantitatively and selectively the carboxyl terminal octapeptide from porcine insulin B chain. This fragment can be replaced by an analogous synthetic human octapeptide to give a protein which is identical to human insulin by a number of criteria. By this method, human insulin can be prepared on a large scale simply and inexpensively from porcine insulin. The method is also useful for preparing specifically labeled radioactive human insulin, as well as insulins with modified amino acid sequences, for research purposes.

Insulin levels in primates by immunoassay

Only trace amounts of insulin were detected by an immunoassay system with guinea pig antibody to pork insulin in the New World primates Cebus and Saimiri. The system found insulin levels in the Old World primates rhesus and chimpanzee which were quite like those of human beings. The findings suggest important structural differences in the insulins of the two primate divisions.