Tryptophan, corn plants

Proteins that are severely deficient in one or more of the essential amino acids are called incomplete proteins. If the protein in a person s diet comes mostly from one incomplete source, the amount of human protein that can be synthesized is limited by the amounts of the deficient amino acids. Plant proteins are generally incomplete. Rice, com, and wheat are all deficient in lysine. Rice also lacks threonine, and corn also lacks tryptophan. Beans, peas, and other legumes have the most complete proteins among the common plants, but they are deficient in methionine. 

Nicotinamide-adenine dinucleotide (NAD diphosphopyri-dine nucleotide) and nicotinamide-adenine dinucleotide phosphate (NADP also termed triphosphopyridine nucleotide) represent most of the niacin activity found in good sources that include yeast, lean meats, liver, and poultry. Milk, canned salmon, and several leafy green vegetables contribute lesser amounts but are still sufficient to prevent deficiency. Additionally, some plant foodstuffs, especially cereals such as corn and wheat, contain niacin bound to various peptides and sugars in forms nutritionally not readily available (niacinogens or niacytin). Because tryptophan is a precursor of niacin, protein provides a considerable portion of niacin equivalent. As much as two thirds of niacin required by adults can be derived from tryptophan metaboHsm via nicotinic acid ribonucleotide... 

Tryptophan is the least abundant amino acid in most proteins,40 accounting, on the average, for 1 to 1.5% of the total amino acids in typical plant (1%) and animal (1.5%) proteins. A number of foodstuffs, such as corn, are deficient or limited in tryptophan. Because it is present in low concentrations in most tissue proteins, the requirement of tryptophan in the diet is low compared to that of the other amino acids, particularly the other indispensable (essential) amino acids. In human infants, the requirement for growth is roughly 12 to 40 mg/kg. In adult humans, the minimum daily requirement has been estimated to be 250 mg/d in males and 160 mg/d in females.41 Considering the recommended daily allowance for protein is 56 g/d for an adult man and 44 g/d for an adult woman, then this amount would supply between 500 and 700 mg/d of tryptophan, assuming that the protein was of high quality. A typical western diet may supply approximately 600 to 1200 mg L-tryptophan from protein intake.42... 

Individual measurement of all five tryptophan pathway enzymes have been reported from tobacco and carrot cell cultures, wheat, corn, and peas (Widholm, 1973 Singh and Widholm, 1974 Hankinsa/, 1976). The relative amounts of the enzymes assayed in vitro differ with the various sources. However, Singh and Widholm (1974) reported extractable quantities of each enzyme in wheat, regardless of the tissue source or plant age, sufficient to synthesize the amount of tryptophan present within the same tissue in 48 h. No in vitro aggregation of any of the tryptophan branch enzymes was ob-... 

Glyphosate, the active ingredient in a well-known herbicide called Roundup, kills weeds by inhibiting an enzyme that plants need to synthesize phenylalanine and tryptophan, amino acids they require for growth. Corn and cotton have been genetically engineered to tolerate the herbicide. Then, when fields are sprayed with glyphosate, the weeds are killed but not the crops. 

For years, plant scientists assayed the world s corn varieties one by one, looking for a strain with more nutritionally balanced protein. Finally, in 1963, a Purdue University team headed by biochemist Edwin T. Mertz analyzed an odd group of corns characterized by soft, floury endosperm inside an opaque, chalk-white kernel. The Purdue scientists found that the opaque characteristic of corn, which had been noted for years without exciting much scientific interest, is associated with a recessive gene that replaces some of the kernel s amino acid deficient zein with other protein higher in the needed lysine and tryptophan. The mutant— routinely labeled opaque-2, or O2 for short—had a lysine... 

---