Apart from
immunoglobulin A and G
antibodies and plasma cells, milk also contains
antibiotic/host protective
peptides that are of value not only for maintenance of its nutritional integrity but also for protection of the newborn and, possibly, protection of the lactating mother. Among the first such
peptides identified with casecidin; following
chymosin digestion of
casein at pH 6 or 7, casecidin inhibited in vitro staphylococci, sarcina, Bacillus subtilis, Diplococcus pneumoniae and Streptococcus pyogenes. Inhibition occurred at high concentrations, in vitro, compared with commercial
antibiotics, and thus interest in casecidin languished. Work with casecidin was followed by investigation of a related refined non-immunogenic product of
chymosin digestion of alpha s1-casein. This product consisted of the N -terminal segment (1-23) of alpha s1-casein B, named "
isracidin", and was significantly effective in vivo at concentrations that were competitive with known
antibiotics, as seen in the protection of mice against lethal
infection by Staphylococcus aureus strain Smith. Field trials showed that injection of
isracidin into the udder gave protection against
mastitis in sheep and cows.
Isracidin was both therapeutic and prophylactic and responses to its
therapeutic effect produced long-term immune resistance.
Isracidin protected mice against Candida albicans, by stimulation of both phagocytosis and immune responses. However, like other recently described milk-derived
peptides, despite its clinical value,
isracidin was overlooked because of the lack of commercial interest in the 1970s and early 1980s, in host-mediated non-specific resistance as a therapeutic approach to
infection. Another problem that impeded commercial interest was the isomeric variation in
isracidin peptides seen on a large-scale batch production for commercial use. It is hoped that this review of previous studies of the activity of
isracidin action will revive interest in milk as an
antibiotic source.