Oxidation rates of Krebs cycle carboxylic acids by the mitochondria of hypotonically treated rabbit epididymal spermatozoa

Oxygen uptake by mitochondria in demembranated human spermatozoa: a reliable tool for the evaluation of sperm respiratory efficiency

In this work we report a relatively simple and fast method for analysing oxygen consumption and therefore mitochondrial functionality, in individual human ejaculates. This oxygraphic method requires a low number of cells, is highly reproducible and linearly correlates with sperm concentration. Our results have shown that oxygen uptake by mitochondria of demembranated sperm cells from normozoospermic subjects is significantly stimulated by a large set of respiratory substrates and ADP. The respiratory control ratio (RCR) values indicate a good coupling between respiration and phosphorylation by sperm mitochondria and thus a well preserved integrity of the mitochondria themselves. Interestingly, whereas the rates of oxygen uptake, as expected, changed with different sperm concentrations, the RCR values remained constant, thus demonstrating a linear response of the assay. In asthenozoospermic subjects, however, a significant decrease in the sperm respiratory efficiency was found. The results obtained suggest that this method, besides its potential clinical application, could be useful for a deeper understanding of the biochemical properties of sperm mitochondria and their role in ATP production in human spermatozoa.

Catalytic properties of the sperm-specific lactate dehydrogenase (LDH X or C4) from different species

A comparative study of catalytic properties of the sperm-specific lactate dehydrogenase (EC 1.1.1.27) isozyme X or C4 from a variety of animals (boar, bull, goat, Guinea pig, man, mouse, pigeon, rabbit, and rat) is presented. Optimum concentration and Km values for pyruvate, inhibition by substrate, and activity against analog substrates (alpha-ketoacids with linear and branched chains from 4 to 6 carbon atoms) for isozyme X of different species showed significant differences. The observed properties are correlated with available evidence on the metabolic role of the enzyme.

Studies in vitro on shuttle systems of mouse spermatozoa

Observations on systems reconstituted in vitro with different starting substrates (2-hydroxy-acids, 2-oxo-acids or leucine) indicate that a branched-chain 2-hydroxy-acid/2-oxo-acid shuttle for the transfer of reducing equivalents from cytosol to mitochondria may be operational in mouse sperm. Evidence is presented suggesting that the 2-oxo-acids produced by intramitochondrial oxidation of 2-hydroxy-acids ingressed from the cytosol can recycle back into the external phase. Observations in vitro demonstrate that, in addition to the branched-chain 2-hydroxy-acid/2-oxo-acid shuttle, the malate/aspartate system is also active in mouse sperm. On the contrary, the lactate/pyruvate redox couple does not appear to function as part of a shuttle system in mouse sperm mitochondria. The glycerol 3-phosphate shuttle probably is not functionally significant in mouse spermatozoa, since the activity of the 'soluble' glycerol 3-phosphate dehydrogenase is very low.

Oxidative metabolism of spermatozoa from inbred and random bred mice

Epididymal spermatozoa from the random-bred CFW and from the inbred C57 BL/6 strains of mice were treated either hypotonically or with the antibiotic filipin in order to study the mitochondrial oxidative activities of the two strains in the absence of permeability barriers imposed by the plasma membrane. The percentage of motile spermatozoa from C57BL/6 mice was consistently higher than that of CFW mice, but sperm from the latter fertilized a higher percentage of eggs in vitro. In vivo, there was no apparent difference in fertilizing capacity in vivo: no significant difference between strains was observed. There is a strategy of oxidative metabolism in mouse spermatozoa which is common to the two genetic strains of the species tested, but which differs from that of rabbit and bull spermatozoa. The mitochondria of mouse spermatozoa oxidize L--3-glycerolphosphate but not glutamate in the presence of malate; both activities are present in bull spermatozoa but neither are present in rabbit spermatozoa. In common wit those of the mammalian species thus far studied, the mitochondria of mouse spermatozoa readily oxidize lactate and pyruvate in the presence of malate. They also oxidize acetyl CoA, acetyl carnitine, and long-chain acyl CoA esters directly, without the intermediacy of the carnitine esters. Mouse spermatozoa, therefore, have access to endogenous acyl CoA esters as a source of metabolic energy, which is consistent with their ability to maintain motility for 4-6 hours in the absence of added energy sources. Mouse spermatozoa are self-sufficient with regard to oxidative metabolism, which suggests that energy sources are not readily available to them in the mouse female reproductive tract.