Milk proteins have largely been considered as providing essential amino acids, but oligopeptides derived from milk proteins have been shown to possess biological functions. In vitro, opioid activity was first reported in bovine β-casein hydrolysate by Brantl et al. (1979). Precursors of biologically active peptides have been demonstrated in vivo after digestion of milk (Scanff et al. 1992). Peptides that inhibit platelet aggregation (Fiat et al. 1989), stimulate the immune system (Migliore-Samour et al. 1989), inhibit angiotensin I converting enzyme (Maruyama & Suzuki, 1982) and are involved in intestinal Ca solubilization and absorption (Sato et al. 1986) have also been isolated from bovine casein hydrolysates.

Bioactive peptides from whey proteins and their physiological effects have received less attention than those from casein. Peptides with opiate-like activity include the lactorphins, residues 50–53 in bovine and human α-lactalbumin and 102–105 in bovine β-lactoglobulin (β-lg) (Chiba & Yoshikawa, 1986; Yoshikawa et al. 1986; Antila et al. 1991). Yamauchi (1992) has reported that a peptide derived from β-lg induced contraction of guinea pig ileum longitudinal muscle in the absence of electric stimulation and agonist. The fragment, containing residues 146–149 of β-lg (His–Ile–Arg–Leu), was called β-lactotensin.

The purpose of this study was to determine whether β-lactotensin was released from bovine β-lg by in vitro proteolysis using different proteolytic enzymes. The pharmacological activity of this tetrapeptide was characterized in guinea pig ileum in vitro using a synthetic fragment.