Regulation of Urea Production in Evernia prunastri: Effects of L-Arginine Metabolites

Vicente, Carlos; Legaz, Marı́a Estrella

doi:10.1016/s0044-328x(83)80038-5

Cited by 11 publications

(1 citation statement)

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“…There are certain similarities and differences between the two forms ofthe enzyme, in addition to the different optimal pH values and mol wt previously described. For instance, agmatine behaves as a noncompetitive inhibitor of the inducible arginase (26), whereas it is an activator ofthe constitutive enzyme. In contrast, both forms ofthe enzyme are activated by putrescine and L-ornithine and inhibited by urea, although the latter is a competitive inhibitor of the inducible arginase and a mixed inhibitor of the constitutive form.…”

Section: Discussionmentioning

confidence: 99%

Purification and Properties of the Constitutive Arginase of Evernia prunastri

Martín-Falquina¹,

Legaz²

1984

Plant Physiol.

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Constitutive arginase (molecular weight 330,000) 920-fold purified from Evernia prunastri thallus, is activated by putrescine, L-ornithine, and agmatine with K. values of 2.7, 1.1, and 5.8 millimolar, respectively. Constitutive arginase is also activated by endogenous L-arginine, reaching its maximum activity at 16 hours of incubation on Tris-HCI (pH 9.15) with a subsequent decrease. Urea behaves as a mixed inhibitor of the enzyme with a K, value of 2.6 millimolar. Atranorin and evernic acid behave as in vitro activators of the enzyme; usnic acid does not have any significant effect as activator.is followed by a loss of enzyme activity after a certain time of incubation. In the latter species, a decrease in activity ofenzymes of the arginine catabolism-arginase, L-arginine decarboxylase, and agmatine amidinohydrolase-has been reported, which is parallel by the accumulation ofboth chloroatranorin and evernic acid (13). The regulation of both phenolics depends, not only on its concentration, but also on the enzyme involved.This paper reports the purification and properties of the constitutive arginase in E. prunastri thallus, as well as the role that lichen phenolics could play on the arginase regulation, in a similar way to that described for the inducible arginase (13). MATERIALS AND METHODSFree L-arginine accumulates in the thallus of many lichen species (18,19) and it is the most abundant free amino acid in Evernia prunastri thalli (11), although seasonal variations in the arginine concentration have been reported. L-Arginine can be hydrolyzed by arginase to produce L-ornithine and urea (10) or decarboxylated by L-arginine decarboxylase to produce agmatine (24), which is later hydrolyzed to give putrescine and urea (25). The main route in E. prunastri thallus is the hydrolytic one, since arginase has greater affinity for Larginine than L-arginine decarboxylase.Arginase synthesis is induced by L-arginine in E. prunastri thallus incubated in the dark (10). It has a mol wt of 180,000 and a pH optimum at 9.1. The Km value has been estimated as 0.2 mm for L-arginine. L-Ornithine and putrescine behave as activators of the enzyme (Km = 0.13 and 0.14 mm, respectively) (10). Agmatine is a noncompetitive inhibitor (Ki = 21.5 mM) and urea is an uncompetitive one (Ki = 2.6 mM) (10).The enzyme is also induced in Escherichia coli (5), Lactobacillus lactis (1), rat kidney (9), and human liver (3). But, in Neurospora crassa, arginase activity is not dependent on protein synthesis (28, 29), being activated by the liberation of L-arginine to the cytosol.In germinating seeds, arginase is dependent on the mobilization of reserve proteins with high content of L-arglnine, as reported in Cucurbita (20), Vicia faba (2), and Pisum sativum (17).A second form of arginase, the constitutive one, reported in E. prunastri thallus is an inactive and preexistent protein which is activated by the liberation of L-arginine to the cytosol. It has a mol wt of about 330,000 and a pH optimum at 6.5 (12

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Section: Discussionmentioning

confidence: 99%