The decomposition of trimethylamine oxide (TMAO) to dimethylamine (DMA) was studied in the mantle of squid, Illex illecebrosus. The production of DMA in either whole or ground mantle stored at – 20°C was low or nonexistent. However, significant DMA was produced from TMAO during heating of squid or squid extracts. Conversion of TMAO to DMA was shown to reside in the soluble fraction of the squid mantle which catalyzed the breakdown of TMAO in an assay system consisting of FeCl2 and ascorbate at room temperature. On the average, some 10–15% of the activity was accounted for by a high molecular weight, thermolabile substance, presumably an enzyme. The rest of the activity was in a small molecular weight, thermostable fraction. The low molecular weight fraction produced significant quantities of TMA as well as DMA, had low activity with flavin‐NADH, and was not activated by methylene blue.
A comparison was made of some kinetic properties of three chicken lactate dehydrogenase isoenzymes (1, 3 and 5) at 4, 16, 23 and 40°C. Assays were performed with an enzyme concentration of 0.01 pM at pH 6.0. Under the conditions of assay, lactate dehydrogenase 3 and 5 bound to the particulate fraction of homogenized skeletal muscle and were evaluated in the soluble and particulate state.Binding of isoenzymes 3 and 5 to the cellular particulate fraction decreased V. This decrease was much greater for lactate dehydrogenase 5 than 3. Values of V for lactate dehydrogenase isoenzymes 1 and 3 did not follow a simple Arrhenius relationship; there was a rapid change in activity between 16 and 23 "C.The apparent K, (pyruvate) values of all isoenzymes (bound or soluble) increased with increasing temperature, changing 4 -10-fold. The apparent K, for lactate dehydrogenase 5 was greater than that for lactate dehydrogenase 3, which in turn was greater than that for lactate dehydrogenase 1.Much interest has focused on the role of isoenzymes in metabolic control. N o enzyme with isoenzymic forms has received more attention in this regard than lactate dehydrogenase. The different distribution of the isoenzymes of lactate dehydrogenase in various tissues has been related to different functions for the different isoenzymes [l -31. Isoenzymes which contain a high content of heart-type (H) subunits are found in aerobic tissues and the isoenzymes with a predominance of M or muscle-type
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