Tissue‐specific alanopine dehydrogenase and strombine dehydrogenase from the sea mouse, Aphrodite aculeata (polychaeta)

Abstract

AbstractEach tissue of the sea mouse, Aphrodite aculeata, contains a single cytoplasmic dehydrogenase acting at the pyruvate branchpoint: Alanopine dehydrogenase (ADH) occurs in longitudinal muscle, nerve, and elytra, and strombine dehydrogenase (SDH) characterizes pharynx and intestine. Lactate and octopine dehydrogenases were not detected in any tissue. ADH from muscle and SDH from pharynx were partially purified. The two enzymes had the same molecular weight, 44,000 ± 4,000 but differed in isolelectric point. Although similar in keto acid specificity, the enzymes differed strongly in amino acid specificity. Pharynx SDH showed a very high specificity for glycine, all other alternative amino acids showing very high apparent Kms and/or low Vmaxs. Muscle ADH, however, displayed high velocities and relatively low apparent Kms with L‐alanine, L‐serine, L‐threonine, and L‐cysteine. Amino acid analysis of the tissues showed that serine and threonine contents were high enough in muscle to suggest that these amino acids might be alternative substrates to alanine in vivo; only glycine, present at 382 μmol/g in pharynx, is likely to be a physiological substrate for SDH, however. Absolute Kms for L‐alanine, glycine, and pyruvate were 2.75 ± 0.03, 285 ± 6, and 0.2 ± 0.006 for muscle ADH and 130 ± 2.2, 26 ± 1 and 0.15 ± 0.005 for pharynx SDH. In the reverse direction, apparent Kms for meso‐alanopine, D‐strombine, and NAD, at pH 9.5, were 1.68 ± 0.06, 87.0 (n = 1), and 0.75 ± 0.03 for ADH and 6.75 ± 0.20, 7.57 ± 0.07, and 0.54 ± 0.02 for SDH. The enzymes also differed strongly in effects of inhibitors. Meso‐alanopine, L‐lactate, and D‐lactate were strong inhibitors of the forward reaction for muscle ADH. Pharynx SDH, however, was strongly inhibited by meso‐alanopine and iminodiacetic acid but showed only weak inhibition by D‐ or L‐lactate or by its product, D‐strombine.