The nutritional toxicity of sweet lupin (Lupinus angustifolius) seed proteins

The effects of raw sweet lupin (Lupinus angustifolius) meal on the growth a nd N utilisation of rats were determined in two ad libitum and two restrict ed-feeding net protein utilisation (NPU) and five N balance experiments. Sw eet lupin seed grown in Western Australia, obtained as meal, either unsuppl emented (LMU) or fully supplemented with required amino acids (360 g kg(-1) ) (LMFS), was tested. Rats fed lactalbumin (130 g kg(-1)) (LACT) were used as positive controls, while rats fed a nonprotein diet (NPC) were used as n egative controls. In addition, seed protein, extracted at pH 7.0 with water and insoluble after dialysis at pH 7.0 (LPADI; 124 g kg(-1)), was also use d. The diets contained the same amounts of energy and protein and were supp lemented with essential amino acids, vitamins and minerals to target requir ements for rats. Inclusion of LPADI in the diet of growing rats caused urin ary losses of N, almost all as urea, hypoproteinaemia and increase in body water that resulted in the lowest NPU values, N balance and growth rate as compared with other diets used. These rats developed atrophy of the spleen (low dry weight) and had a comparatively smaller thymus gland than those gi ven raw meals. Furthermore, the LPADI fraction was shown by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) to contain three pol ypeptides with molecular weights between 30 and 36kDa which are similar to lectins obtained from Phaseolus vulgaris, Abrus precatorius and Ricinus com munis. It is possible that the toxic protein component in the sweet lupin, which has negligible ie vitro haemagglutination properties and is extremely toxic ill vivo, exerts toxicity by interfering with protein synthesis in t he liver, while the immune responses are secondary to azotaemia (high level of urea in the blood) or cytotoxicity action on lymphocytes. The unusual d epletion of fat from the body, however, was due to the failure of absorbed amino acids to assimilate as proteins, creating dietary protein restriction and leading to lipolysis. It is therefore tentatively suggested that sweet lupin seed contains a lectin-like protein that is concentrated in this fra ction. Further purification and biological evaluation to establish the exac t nature of this protein may be important. (C) 2000 Society of Chemical Ind ustry.