Asparagine breakdown

In 1980, Lea and Miflin discussed the transport and metabolism of asparagine and other nitrogen compounds within the plant. Since that time there have been major developments in the study of asparagine breakdown but the precise enzymology of asparagine synthesis in green leaves has still not been established (Sieciechowicz et al, 1988a). 

An apparently more effective method for prolonging the half-life of insulin in the blood is to add substituents at the end of the A- or B-chain (or both) that alter the chemical properties of the molecule and delay its breakdown in the body. A product known as HOE 901 (insulin glargine) has two glycine residues added to one end of the B-chain and the A21 asparagine residue replaced with another glycine residue. These changes modify the acidity of the insulin molecule, reducing the rate at which it is absorbed and metabolized in the body. 

The "chemical noise" problem requires the breakdown of proteins into polypeptides through the use of enzymes of known specificity. For example, trypsin is known to cleave proteins at lysine or asparagine residues peptides thus produced may be sequenced and a "puzzle" is thus produced which must be solved by laying sequences over one another and the use of chemical insight to fit the pieces together. 

Acid hydrolysis yields 16 of the 20 coded amino acids tryptophan is destroyed, cysteine recovery is unreliable, and asparagine and glutamine are converted to aspartic acid and glutamic acid, respectively. Furthermore, some side groups, such as the hydroxyl in serine, promote the breakdown of the residue, whereas aliphatic amino acids, protected by stearic hindrance, require longer hydrolysis time. This variation in yield can be overcome by hydrolyzing samples for 24, 48, and 72 h and extrapolating the results to zero time point. 

Joy and his colleagues have also monitored the fate of the asparagine carbon skeleton. Almost 75% of the [ CJasparagine supplied was metabolized in 210 min in the light but only 45% in the dark. Over 50% of the metabolized asparagine accumulated in a novel compound 2-hydroxysuccinamate (see Section III,C,2). 2-Oxosuccinamic acid was shown to be a precursor of 2-hydroxysuccinamic acid. Other amino acids were also labeled particularly aspartate, with lesser amounts in glutamate, homoserine, and alanine. 2-Hydroxysuccinamate was only slowly metabolized (approximately 20% in 210 min) although there was evidence of more rapid breakdown in the dark (Lloyd and Joy, 1978). 

The transamination of asparagine in soybean leaf extracts was detected by Streeter (1977). Glyoxylic acid was the most active 2-oxo acid acceptor, followed by pyruvate, oxaloacetate, and 2-oxoglutarate. The breakdown of the product 2-oxosuccinamic acid to oxaloacetate and ammonia was also detected. 

Lloyd and Joy (1978) have confirmed the transmination of asparagine as a major pathway of the breakdown of the amide in leaves. The product 2-oxosuccinamate may be deaminated, but the majority was reduced to 2-hydroxysuccinamate which tended to accumulate. Very low levels of activity of the asparagine aminotransferase were detected in maturing pea seeds compared to the K+-activated asparaginase (K. W. Joy, personal communication). 

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