Effect of heat processing on in vitro protein digestibility and some chemical properties of African breadfruit (Treculia africana Decne) seeds.

The effects of dry heat (roasting) and moist heat (boiling) on in vitro protein digestibility, protein fractions and other chemical properties of African breadfruit (Treculia africana Decne) seed that affect their utilization as a source of human food were investigated. Chemical analyses showed that the crude protein and fat contents of the unprocessed (raw) seeds were 20. 1% and 13.7%, respectively. The level of phytic acid in the raw seed (1.19 mg/g) was lower than the levels found in some commonly consumed pulses in Nigeria. Albumin and globulin protein fractions were found to be the major seed proteins of African breadfruit seed, constituting 67.8% of the total protein of the raw seed. There were no significant (p > 0.05) differences between crude protein, ash and fat contents of the raw and heat processed samples. Boiling proved more effective than roasting for improving protein digestibility and for reducing the levels of trypsin inhibitor, phytic acid and polyphenols of the samples. The complete removal of these antinutrients, however, would require a more severe heat treatment of the seed, which in turn would profoundly reduce the nutritional value and availability of proteins, as demonstrated by the low values obtained for in vitro protein digestibility, protein fractions and protein extractability.

Enzymatic Basis for Overproduction of Tryptophan and Its Metabolites in Hansenula polymorpha Mutants.

3-Deoxy-d-arabinoheptulosonate 7-phosphate (DAHP) synthetase and anthranilate synthetase are key regulatory enzymes in the aromatic amino acid biosynthetic pathway. The DAHP synthetase activity of Hansenula polymorpha was subject to additive feedback inhibition by phenylalanine and tyrosine but not by tryptophan. The synthesis of DAHP synthetase in this yeast was not repressed by exogenous aromatic amino acids, singly or in combinations. The activity of anthranilate synthetase was sensitive to feedback inhibition by tryptophan, but exogenous tryptophan did not repress the synthesis of this enzyme. Nevertheless, internal repression of anthranilate synthetase probably exists, since the content of this enzyme in H. polymorpha strain 3-136 was double that in the wild-type and less sensitive 5-fluorotryptophan-resistant strains. The biochemical mechanism for the overproduction of indoles by the 5-fluorotryptophan-resistant mutants was due primarily to a partial desensitization of the anthranilate synthetase of these strains to feedback inhibition by tryptophan. These results support the concept that inhibition of enzyme activities rather than enzyme repression is more important in the regulation of aromatic amino acid biosynthesis in H. polymorpha.

Derivation of Aromatic Amino Acid Mutants from a Methanol-Utilizing Yeast, Hansenula polymorpha.

Three classes of mutants, deregulated to enhance the flow of aromatic intermediates through the tryptophan biosynthetic branch, were obtained. 5-Fluorotryptophan, an antimetabolite of tryptophan, was employed to obtain one class of deregulated mutants. By sequential resistance development, three resistant mutants were isolated. Hansenula polymorpha strains showed greater sensitivity to 5-fluorotryptophan when growing on methanol than when growing on glucose. Yeast extract stimulated the production of total indole metabolites (indoles) by wild-type and mutant strains, with each 5-fluorotryptophan mutant producing higher amounts of these metabolites than its predecessor. Two other mutant classes were isolated: (i) a mutant resistant to anthranilate (an inhibitory intermediate in the tryptophan biosynthetic branch) and (ii) a phenylalanine-plus-tyrosine bradytroph. Each of these produced a higher extracellular titer of total indoles than its immediate parent. With respect to the overproduction of indoles, resistance to 5-fluorotryptophan was a more useful selection method than were anthranilate resistance and phenylalanine-plus-tyrosine bradytrophy.