THE RELATIONSHIP OF ARGININE GROUPS TO PHOTOSYNTHETIC HCO3- UPTAKE IN ULVA SP. MEDIATED BY A PUTATIVE ANION-EXCHANGER

The marine macroalga Ulva sp. can take up HCO3- via a process which chemically resembles that of anion exchange in red blood cells (Drechsler et al. 1993, Planta 191, 34-40). In this work we explore the possibility that high-pK amino-acid residues could be functionally involved in the binding/transport of HCO3-. It was found that the specific arginyl-reacting agents phenylglyoxal and 2,3-butanedione inhibited photosynthesis of Ulva competitively with inorganic carbon at pH 8.2-8.4 (which is close to the pH of normal seawater), where HCO3- was the predominant inorganic carbon form taken up. The inhibition by phenylglyoxal was irreversible at 32 degrees C and high pH values, while that of butanedione became irreversible in the presence of berate. These interactions, as well as the protection of the irreversible phenylglyoxal-inhibition by inorganic carbon and by the membrane-impermeant agents 4,4'-diisothiocyanostilbene-2,2'-disulfonate and 4,4'-dinitrostilbene-2,2'-disulfonate indicate that arginine (and possibly also lysine) are involved in the HCO3- uptake process, probably at the plasmalemma level. The photosynthetic affinity of Ulva to external inorganic carbon gradually decreased with increasing pH from 8.2 to 10.5, and this decrease parallels the decline in protonation of amino acids with a pK of around 10. Based on this information, as well as the inhibition studies, it is suggested that arginine and lysine residues are essential proteinaceous constituents involved in anionic inorganic carbon (HCO3- and possibly also C0(3)(2-)) uptake into the Ulva cells.