Anaerobic metabolism

Despite their sophisticated adaptations for the protection of aerobic metabolism, cephalopods regularly have to face situations when the O2 supply is insufficient (e.g.

environmental hypoxia). Under these conditions cephalopods can resort to a well-developed anaerobic metabolism to overcome such oxygen-limited situations (Storey

& Storey 1979, Pörtner 1987, Grieshaber et al. 1994). Energy demands during fast jet-propelled swimming usually exceed the energy provision by aerobic metabolism, as this is limited by the O2-supply (Hoeger et al. 1987, Pörtner 1994, Finke et al.

1996, see 1.2 & 1.2.1). A similar situation occurs during environmental hypoxia, when the energy demands are stable, but the O2 supply is reduced and less ATP can be produced.

Anaerobic metabolic pathways help to resolve this impairment of energy demands, albeit creating an oxygen debt due to the accumulation of end products (Lewis et al.

2007, Rosa & Seibel 2008). Anaerobic metabolism in cephalopods is based on the use of glycogen and phospho-L-arginine (PLA) and creates the anaerobic end product octopine + H⁺ (Fig. 1.3) (Grieshaber & Gäde 1976, Pörtner 1987). The proton actually derives from pyruvate formed during anaerobic glycolysis. The pyruvate is condensed with L-arginine from phosphagen mobilization yielding octopine, which means that per mol octopine (or pyruvate), 1 mol H⁺ is produced (Grieshaber & Gäde 1976, Pörtner 1987). The production of protons supports the mobilization of the phosphagen and the transfer of the phosphate group yielding ATP. Although the phosphagen mobilization consumes protons (0.24 mol H⁺ per 1 mol PLA at pH 7.3, Pörtner 1987), the anaerobic metabolism causes a drop in intracellular pH (pHi), which is clearly correlated to the production of octopine + H⁺ (Pörtner et al. 1991, Pörtner et al. 1993). Other accumulating end products are α-glycerophosphate and NADH+H⁺ from the anaerobic glycolysis (Fig. 1.3). Due to the lack of oxygen, the mitochondrial ETS can no longer consume reduction equivalents and NADH+H⁺ is no longer shuttled into the mitochondria via α-glycerophosphate (Fig. 1.3) (Grieshaber &

Gäde 1976, Pörtner 1987). Generally, anaerobic pathways provide less ATP than aerobic ATP production by the ETS. The aerobic metabolization of 1 mol glucose to CO2 + H2O provides ~36 mol ATP + ~18 additional ATP, if proline is metabolized in parallel (Hochachka et al. 1975, Storey & Storey 1983). The anaerobic degradation to intermediates like succinate does not create more than 4 mol ATP / mol glucose (Hochachka et al. 1975, Storey & Storey 2005). Therefore, glycolytic enzyme activity

is elevated during anaerobic metabolism to increase the ATP output (Finke et al.

1996). Anaerobic metabolism is time-limited due to the depletion of energy storages (glycogen, PLA) and the accumulation of end products (Pörtner 1987).

The net degradation of ATP during anaerobiosis results in a higher total ADP-concentration, as well as a higher percentage of unbound ADP in the cytosol (Pörtner et al. 1993). AMP-concentrations increase too, with a higher fraction of AMP remaining unbound (Pörtner et al. 1993). The increase in free ADP and AMP affects enzymatic functions. It has been shown that free ADP enhances glycolytic enzyme activities (Storey & Storey 1978). The preferred way of PLA mobilization differs between cephalopod species. In the hypoxia-tolerant brief squid Lolliguncula brevis, total and free ADP stay almost constant during anaerobic metabolism and the use of PLA is mostly triggered by a drop in pHi (Pörtner et al. 1996, Pörtner 2002). In contrast, anaerobic metabolism causes only small intracellular acidosis in the longfin inshore squid Loligo pealei and PLA mobilization is mainly caused by a strong rise in free ADP while total ADP stays more or less constant (Pörtner et al. 1993, Pörtner 2002). The accumulating inorganic phosphate (Pi) released during ATP-hydrolysis additionally enhances glycogen mobilization (Fig. 1.3) and thus provides fuel for the glycolysis (Pörtner & Zielinski 1998). The accumulation of NADH+H⁺ and octopine would usually inhibit glycolytic enzyme activities (Storey 1981, Pörtner et al. 1993), but the increase in free AMP overrides this inhibition and activates phosphofructokinase, which is a key enzyme of glycolysis (Storey & Storey 1983, Pörtner et al. 1993). The maintenance of proper glycolytic function during anaerobiosis by high free AMP levels is an exclusive feature of cephalopods (Storey

& Storey 1983).

As indicated by the accumulation of α-glycerophosphate (see above), hypoxia is not confined to the cytosol, but does also affect mitochondria (Pörtner 1987, Finke et al.

1996). During hypoxia, the O2 supply is insufficient to ensure metabolization of Krebs-cycle and ETS intermediates. Thus, the intermediates accumulated and stop both pathways by product inhibition. The onset of anaerobic metabolism in the cytosol also seems to be triggered by mitochondrial hypoxia as shown in L. brevis (Pörtner 1995, Finke et al. 1996). One intermediate of mitochondrial anaerobic metabolism is succinate (Fig. 1.3) and acetate and propionate may accumulate during long-term anaerobic metabolism (Pörtner 1987, Grieshaber et al. 1994). All three metabolites derive from reactions of the Krebs-cycle, which utilize malate and

may also be fueled by proline degradation (Pörtner 1987, Storey & Storey 1978, Mommsen et al. 1982) However, the concentration of succinate during hypoxia has been found to be much lower than that of octopine indicating a minor role of mitochondrial anaerobic ATP production during hypoxia (Zielinski et al. 2000, Rosa &

Seibel 2010). If the ETS-induced membrane potential is maintained, there is no drop in mitochondrial pH during anaerobiosis, as protons are consumed in the Krebs-cycle (Pörtner 1987).

Despite the buffering of ATP levels by the use of PLA storages, anaerobic metabolism ultimately leads to a decrease and ATP and Gibbs free energy. There are several mechanisms to delay the drop of the Gibbs free energy, which were nicely summarized by Pörtner (2002). The accumulation of free ADP supports the buffering of ATP, as it activates glycolysis and thus ATP production. The production of octopine removes arginine, which would antagonize PLA mobilization and thus ATP buffering. The intracellular acidosis and the accumulation of Pi from ATP degradation both decrease muscle performance and thereby also reduce ATP consumption (Pörtner 2002).

The fate of the anaerobic end products is still under discussion. Storey & Storey (1979) found a rapid uptake of octopine injected into the blood by aerobic tissues (brain, ventricle) and proposed a blood transport of octopine from sites of production (anaerobic tissues) to sites of O2-consuming degradation (aerobic tissues). This assumption is supported by the presence of different isoforms of the octopine creating/degrading enzyme octopine-dehydrogenase (ODH) in the different tissues of S. officinalis. Whereas an octopine forming isoform is dominant in anaerobic tissues, an octopine degrading isoform predominates in aerobic tissues (Storey 1977).

However, findings of constantly low blood octopine levels in squids during rest, exercise and recovery (Pörtner et al. 1991) contradict this hypothesis. Pörtner et al.

(1991) found octopine and metabolic protons almost completely retained in the cells of squid mantle tissue during exercise and postulated that anaerobic ends products are recycled within the tissue, when the O2 is again sufficient (Pörtner et al. 1993). As protons do not leave the cellular space, a drop in pHi should not affect pHe. Only if stressful conditions (e.g. exercise, hypoxia) are too severe or last too long, a drop in pHe can be observed indicating an H⁺ leakage from the tissue (Pörtner et al. 1991).