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Metabolism of Bovine Semen. XV. Rate-limiting Reactions in the Citric Acid Cycle¹

Department of Dairy Science, The Pennsylvania State University, University Park

ABSTRACT

Possible rate-limiting reactions in the citric acid cycle were investigated by measuring the evolution of C¹⁴O₂ from labeled intermediates of the cycle. Ejaculated bovine spermatozoa, either washed (W), versene-treated (V), or subjected to ultrasonic vibration (S) were incubated at 37 C in phosphate buffer with labeled -ketoglutarate, succinate, malate, and citrate. The C¹⁴O₂ evolution was greatest with -ketoglutarate as substrate; for comparative purposes it was assigned a relative specific activity (RSA) of 100 and yields from other substrates compared to it. With W, RSA values were 100, 17, 16, and 11; with V, 100, 14, 15, and 11; and with S, they were 100, 80, 79, and 18, for -ketoglutarate, succinate, malate, and citrate, respectively. Low yields from succinate, malate, and citrate in W and V preparations are attributed to limited permeability of the cell membrane to these substrates. The S data indicate that possibly some rate limitation occurs in the succinyl coenzyme A-succinate portion of the cycle.

¹ Authorized for publication on September 1, 1966, as Paper no. 3175 in the journal series of the Pennsylvania Agricultural Experiment Station. Supported in part by U. S. Atomic Energy Commission Contract AT(30-1)-1849.