Camp. B&hem. Physiol., 1971, Vol. 39B, pp. 159 to 161. Pergamon Press. Printed in Oreat Britain

SHORT COMMUNICATION

REGULATION OF LACTIC DEHYDROGENASE IN THE GIANT SCALLOP, PLACOPECTEN MAGELLANICUS

(GMELIN)

P. J. A. O’DOHERTY* and L. A. W. FELTHAM

Department of Biochemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada

(Received 5 October 1970)

Abstract-l. Glutamate and aspartate, and the Krebs cycle intermediates, in contrast to mammalian skeletal muscle, decreased the activity of lactic dehydro- genase in the adductor muscle of the giant scallop.

2. An explanation for these observed effects is offered.

INTRODUCTION

FRITZ (1965) showed that the dicarboxylic amino acids and the Krebs cycle inter- mediates could activate lactic dehydrogenase from rabbit skeletal muscle. In this investigation we examined the effects of these metabolites on lactic dehydrogenase from the adductor muscle of the giant scallop, as previous work in our laboratory showed this enzyme to be of minor importance in glycolysis in the giant scallop (O’Doherty & Feltham, 1971).

MATERIALS AND METHODS Particulate-free supematant was prepared and lactic dehydrogenase assayed, as previous-

ly described (O’Doherty & Feltham, 1971).

RESULTS

The effects of aspartate and glutamate and of the Krebs Cycle intermediates are shown in Tables 1 and 2 respectively. In contrast to the rabbit skeletal muscle enzyme (Fritz, 1965), these metabolites decreased the activity of the lactic de- hydrogenase. Succinate and isocitrate were shown to exhibit competitive inhibition with respect to pyruvate as shown in Fig. 1. It is possible the other intermediates also exhibit competitive Inhibition.

DISCUSSION

Activation of lactic dehydrogenase is advantageous in mammalian skeletal muscle, as it results in less pyruvate being available for conversion to acetyl

* Present address: Charles H. Best Institute, University of Toronto, Toronto 5, Canada.

159

160 P.J. A. O'DOHERTYAND L.A. W. FELTHAM

TABLE I-THE EFFECT OF ASPARTATE AND GLUTAMATE ON ADDUCTOR MUSCLE LACTIC DEHYDROGENASE

Metabolite

bW

Activity (%)

Aspartate Glutamate

0 100 100 0.1 90 93 0.2 78 80 0.3 72 70 0.4 67 64 0.5 60 57

The reaction mixture contained 0.2 mg protein/ml of adductor muscle super- natant.

coenzyme A and more NAD being available to spark glycolysis through the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase. Thus, when the Krebs cycle substrates act to reduce the concentration of a substance which is one of their precursors, they are in effect controlling their own concentrations, a simple case of feedback control by an end-product of a synthetic pathway. In mammals, the conversion of pyruvate to lactate is the chief means by which cytoplasmic NADH is oxidized.

In scallop muscle, however, the activity of lactic dehydrogenase is low, and lactate is not the end-product of anaerobic glycolysis. In addition, cytoplasmic NADH can be oxidized by cytoplasmic malate dehydrogenase and by the Biicher shuttle (O’Doherty & Feltham, 1971). It, therefore, would be of no advantage to the adductor muscle to accumulate lactate, a compound which it may not be able to metabolize at any appreciable rate. In fact, by decreasing the lactic dehydrogenase activity, it is possibly ensuring a channelling system away from lactate and on to succinate, and, on the way, oxidizing NADH at the malate dehydrogenase level.

TABLE %--EFFECT OF KREBS CYCLE INTERMEDIATES ON LACTIC DEHYDROGENASE

Activity (%) Metabolite

k-W Citrate Isocitrate a-Ketoglutarate Succinate Fumarate Malate

0 100 100 100 100 100 100 0.1 96 93 96 93 93 93 0.2 86 90 80 83 78 78 0.3 78 80 72 78 67 72 0.4 67 75 67 67 62 64 0.5 60 67 57 62 55 57

The reaction mixture contained 0.2 mg protein/ml of adductor muscle super- natant.

LACTIC DEHYDROGRNASE IN THE GIANT SCALLOP 161

7-

6- . 0.2 mM Succinate

0.2 mM lsocitrate

Control

I -2 0 2 4 6 s

L I ’

M-’ x IO-~

Fig. 1

Acknowledgements-We wish to thank Dr. F. A. Aldrich of the University’s Marine Sciences Research Laboratory for all the facilities granted, and the Fisheries Research Board of Canada for financial support.

REFERENCES FRITZ P. J. (1965) Rabbit muscle lactic dehydrogenase: A regulatory enzyme. Science 19,

364-366. O’DOHBRTY P. J. A. & FELTHAM L. A. W. (1971) Glycolysis and glyconeogenesis in the

giant scallop, Placopecten magellanicus (Gmelin). Comp. Biochem. Physiol. 38, 543-551.

Key Word Index-Lactic dehydrogenase; regulatory enzyme; Krebs cycle intermediates.