Isolation of enzymes involved in threonine biosynthesis from sorghum seeds

Abstract

Cereal seeds are poor in essential amino acids, particularly lysine, tryptophan and threonine. The amino acids lysine and threonine are synthesized in the aspartate pathway. Although most of the enzymes of the aspartate pathway have been isolated and characterized in higher plant species, the metabolism of lysine and threonine is totally unknown in sorghum. We have isolated two enzymes, aspartate kinase (AK) and homoserine dehydrogenase (HSDH) from sorghum. Optimum assay conditions were established for the determination of AK and HSDH activities. The highest level of activity was observed in immature seeds. AK was shown to be inhibited by threonine and lysine indicating the existence of at least two isoenzymes, one sensitive to threonine inhibition and the other sensitive to lysine inhibition with the latter being predominant in sorghum seeds. HSDH was shown to be inhibited by threonine indicating the existence of a threonine-sensitive HSDH, however, most of the activity was not inhibited by threonine, suggesting the existence of a second predominant isoenzyme of HSDH resistant to threonine inhibition. Key words: amino acids, aspartate kinase, homoserine dehydrogenase, lysine, threonine.Isolamento de enzimas envolvidas na biossíntese de treonina em sementes de sorgo: As sementes dos cereais são pobres em aminoácidos essenciais, principalmente lisina, triptofano e treonina. Os aminoácidos lisina e treonina são sintetizados na via metabólica do aspartato. Apesar de a maioria das enzimas da via do aspartato ter sido isolada e caracterizada em várias espécies de plantas, o metabolismo de lisina e treonina é totalmente desconhecido em sorgo. Isolamos duas enzimas, aspartato quinase (AK) e homosserina desidrogenase (HSDH) de sorgo, estabelecendo condições ótimas de ensaio para determinação da atividade de AK e HSDH. Observaram-se as atividades mais elevadas de AK e HSDH em sementes imaturas. A atividade de AK foi inibida por treonina e lisina, indicando a existência de duas isoenzimas, uma sensível à inibição por treonina e, a outra, sensível à inibição pela lisina, predominando, esta última, nas sementes de sorgo. A atividade de HSDH foi inibida por treonina, indicando a existência de uma isoenzima sensível à treonina, a maioria da atividade, entretanto, não foi inibida por treonina, sugerindo a existência de uma segunda isoenzima de HSDH predominante resistente à inibição pela treonina. Palavras-chave: aminoácidos, aspartato quinase, homoserina desidrogenase, lisina, treonina.

Isolation and Characterization of Enzymes Involved in Lysine Catabolism from Sorghum Seeds

“…Although extensive information is available about the biosynthetic enzymes such as AK, DHDPS, and HSDH, until recently very little was known about lysine degradation ( − ). Recent studies have indicated that lysine catabolism plays an important role in lysine accumulation in plants and the control of lysine content, particularly of seeds ( , ).…”

Physical Characteristics of Pseudocereals Seeds

“…The amaranth and quinoa crops stand out particularly as a source of good quality protein, which is comparable to milk casein and is present in the grain, leaves, as well as in the inflorescences of plants. Also, its in human food is the absence of gluten, perfectly adapting to the diet of celiac people (FERREIRA, et al, 2004). The nutritional quality of quinoa, for example, is characterized by having lysine, an amino acid absent in cereals, as well as having a better distribution of essential amino acids, as well as high amounts of vitamins such as riboflavin, niacin, thiamine, B6, and minerals such as magnesium, zinc, copper, iron, manganese and potassium (BORGES et al, 2003).…”

PROCESSAMENTO DE BOLO COM FARINHA DE QUINOA (Chenopodium quinoa Willd): ESTUDO DE ACEITABILIDADE

“…O mérito principal da quinoa real é que o grão, as folhas, assim como as inflorescências são fontes de proteína de boa qualidade, que é comparável à caseína do leite. A qualidade nutricional da quinoa real é importante, sendo rica em aminoácidos sulfurados e no aminoácido lisina, ao contrário das proteínas dos cereais, que são deficientes em lisina (Ferreira et al, 2004).…”