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Austromegabalanus psittacus is a giant balanomorph that inhabits subtidal and intertidal rocky shores and resists long periods of anoxia and hypoxia in the laboratory. In the present study, the kinetic properties of the enzyme pyruvate kinase (PK) from opercular muscles of this cirripede are presented, and analysed in association with the animals metabolic capacities for adaptation to its highly fluctuating habitat.The PK of this species showed a high kinetic activity in the tissue. It is an allosteric enzyme, with properties resembling the L-as well as the M-type of PK described for other marine invertebrates. It shows a high affinity for the substrate phosphoenolpyruvate (PEP), with low cooperativity: S 0.5 PEP = 49.9 µM; n H = 1.17 (pH = 7.0). PEP inhibited the enzyme at concentrations greater than 1 mM, with 90% inhibition at 7 mM PEP. This substrate inhibition should not be of physiological relevance. The substrate ADP also showed a high affinity for PK, with hyperbolic saturation kinetics (S 0.5 ADP = 172.1 µM; n H = 1.13). Hexose fructose 1,6-bisphosphate (FBP) activated the muscular PK, lowering the S 0.5 for PEP to 38.5 µM. Nevertheless, the activation of the PK by FBP was lower than that observed for the L-type PKs from molluscs. The optimal pH for PK activity (at physiological PEP and ADP concentrations) was around 7.0, and PK showed similar affinity levels for PEP at pH 6.3 and 7.8, differing from the L-type PKs of facultative anaerobic marine invertebrates (which show higher affinity for PEP with increasing pH levels). ATP inhibited PK (non-competitive inhibition) and the inhibitory effect was more pronounced at lower pH values. At physiological ATP concentration in opercular muscles of A. psittacus (7.00 mM), and in the presence of nearly physiological concentrations of ADP and PEP (1.86 mM and 0.57 mM, respectively), the PK showed no catalytic activity at pH 6.3 and 6.9, and about 10% activity at pH 7.8 (in relation to V max at this pH).The participation of barnacle muscular PK in metabolic control under low oxygen availability periods is discussed. 1 ) RESUMENAustromegabalanus psittacus es un cirripedio balanomorfo que alcanza gran tamaño. Este organismo habita el submareal como también zonas rocosas intermareales, y resiste períodos prolongados de emersión y anoxia. En este trabajo se presenta las características cinéticas de la enzima piruvato quinasa (PK) de músculo opercular del balánido, en asociación con las capacidades metabólicas de adaptación de este organismo bajo las esperadas fluctuaciones ambientales en su hábitat.La PK presenta alta actividad catalítica en músculo de A. psittacus. Es una enzima alostérica que comparte propiedades de las formas tipo L y M de PK de otros invertebrados marinos. Posee alta afinidad por el sustrato fosfoenolpiruvato (PEP), con S 0.5 de 49,9 µM, y una baja cooperatividad para PEP (n H = 1,17). La enzima muestra inhibición por exceso de PEP, a concentraciones de sustrato mayores a 1 mM; esta inhibición no tendría relevancia fisiológica. El sustrato ADP tamb...

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