OXIDATIVE AND HYDROLYTIC ENZYMES IN THE NEPHRON OF NECTURUS MACULOSUS : Histochemical, Biochemical, and Electron Microscopical Studies

The distribution of oxidative and hydrolytic enzyme activities along the nephron of Necturus maculosus Rafinesque was studied histochemically. The proximal tubule possessed all the demonstrable enzyme activities associated with the hexose-monophosphate shunt and glycolysis, but lacked detectable succinic dehydrogenase and cytochrome oxidase activities. Krebs cycle enzymes other than succinic dehydrogenase were easily detectable. The distal tubule, on the other hand, possessed no detectable hexose-monophosphate shunt enzyme activities, but all demonstrable glycolytic and Krebs cycle enzymes and cytochrome oxidase were present in high activity. These data indicate that the proximal tubule of Necturus probably cannot depend, as can the distal tubule, on the Krebs cycle and cytochrome system to provide energy for its transport processes, an inference supported, in general, by available physiological evidence. The question of the importance of the hexose shunt to proximal tubular function arises. Evidence is presented that the proximal tubular blood supply is primarily venous in nature, a hypothesis which would correlate well with its anaerobic metabolic pattern. In addition, the absence of cytochrome oxidase and succinic dehydrogenase from the proximal tubular cells implies either that they possess very few mitochondria, or that their mitochondria have a very unusual enzymatic pattern. Electron microscopical observations and data obtained from the measurement of the enzyme activities of homogenates of Necturus kidney are presented which indicate that the second hypothesis is more probably correct.     

Protein synthesis in the intestine of goldfish acclimatized to different temperatures

1. The incorporation of [¹⁴C]valine and [³H]threonine has been studied in mucosa of goldfish acclimatized to different temperatures. 2. [¹⁴C]Valine was incorporated more rapidly than [³H]threonine at acclimatization temperatures below 15°; this distinction disappeared at higher temperatures. 3. The rate of amino acid incorporation rose as the environmental temperature was increased. 4. The number of valine residues in mucosal protein fell and the number of methionine residues rose as the acclimatization temperature was raised. 5. The concentration of free methionine was closely related to the amount in protein in goldfish mucosa; both increased at higher acclimatization temperatures. 6. An attempt is made to show that these changes are integrated with possible homoeostatic mechanisms in the goldfish intestine.     

Purification and properties of erythro-β-hydroxyasparate dehydratase from Micrococcus denitrificans

1. The novel enzyme, erythro-beta-hydroxyaspartate dehydratase, a key enzyme of the beta-hydroxyaspartate pathway (Kornberg & Morris, 1963, 1965), has been purified 30-fold from extracts of glycollate-grown Micrococcus denitrificans. The purified preparation was devoid of erythro-beta-hydroxyaspartate-aldolase activity, and free from enzymes that act on oxaloacetate. 2. Properties of the purified dehydratase were studied by direct assay of the enzymic formation of oxaloacetate and ammonia from added erythro-beta-hydroxyaspartate. 3. The enzyme was highly substrate-specific, utilizing only the l-isomer of erythro-beta-hydroxyaspartate (K(m), 0.43mm, and V(max.), 99mumoles of oxaloacetate formed/min./mg. of protein at pH9.15 and 30 degrees ). Of many compounds tested, only maleate was a competitive inhibitor (K(i), 32mm at pH7.6). 4. The optimum pH for activity was about 9.5. The K(m) varied with pH, showing a marked optimum at pH7.8. The V(max.) also varied with pH in a manner suggesting the presence in the enzyme-substrate complex of a dissociable group of pK'(a) about 8.5. 5. Carbonyl reagents were inhibitory, but of three thiol reagents tested only p-chloromercuribenzoate was inhibitory. 6. A partially resolved preparation of the enzyme was activated four-fold by the addition of pyridoxal phosphate and thereby restored to half activity. 7. EDTA (0.1mm) was almost completely inhibitory, activity being restored by bivalent cations (Mg(2+), Ca(2+) and Mn(2+)); no activation by univalent cations was observed. 8. The findings are discussed in the light of reported properties of related hydroxyamino acid dehydratases.