The purification, cloning, and expression of a novel luteinizing hormone-induced mitochondrial protein in MA-10 mouse Leydig tumor cells. Characterization of the steroidogenic acute regulatory protein (StAR).

The mechanism regulating the production of steroids in response to trophic hormone stimulation has been the subject of investigation for over three decades. When considering the effects of trophic hormones on the steroidogenic process it is necessary to first distinguish between acute effects and chronic effects. Acute effects are those which result in the very rapid (within minutes) synthesis and secretion of steroids in response to hormone stimulation and involve the rapid translocation of intracellular cholesterol to the site of its cleavage, as will be discussed later. Chronic effects are those which occur on the order of hours to tens of hours and involve increased gene transcription and translation of the proteins involved in the biosynthesis of steroids. This chapter will focus on studies designed to elucidate the mechanisms involved in the acute regulation of steroid production in response to hormone stimulation. Overviews of the effects of chronic stimulation on steroidogenic enzymes have appeared in several excellent review articles (Simpson and Waterman 1983; Miller 1988; Hanukoglu 1992).

Regulation of the acute production of steroids in steroidogenic cells.

The primary function of the corpus luteum is secretion of the hormone progesterone, which is required for maintenance of normal pregnancy in mammals. The corpus luteum develops from residual follic...

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.1.1

Mechanisms Controlling the Function and Life Span of the Corpus Luteum

Steroid hormone biosynthesis is acutely regulated by pituitary trophic hormones and other steroidogenic stimuli. This regulation requires the synthesis of a protein whose function is to translocate cholesterol from the outer to the inner mitochondrial membrane in steroidogenic cells, the rate-limiting step in steroid hormone formation. The steroidogenic acute regulatory (StAR) protein is an indispensable component in this process and is the best candidate to fill the role of the putative regulator. StAR is expressed in steroidogenic tissues in response to agents that stimulate steroid production, and mutations in the StAR gene result in the disease congenital lipoid adrenal hyperplasia, in which steroid hormone biosynthesis is severely compromised. The StAR null mouse has a phenotype that is essentially identical to the human disease. The positive and negative expression of StAR is sensitive to agents that increase and inhibit steroid biosynthesis respectively. The mechanism by which StAR mediates cholesterol transfer in the mitochondria has not been fully characterized. However, the tertiary structure of the START domain of a StAR homolog has been solved, and identification of a cholesterol-binding hydrophobic tunnel within this domain raises the possibility that StAR acts as a cholesterol-shuttling protein.

StAR Protein and the Regulation of Steroid Hormone Biosynthesis

Sulfur is required for growth of all organisms and is present in a wide variety of metabolites having distinctive biological functions. Sulfur is cycled in ecosystems in nature where conversion of sulfate to organic sulfur compounds is primarily dependent on sulfate uptake and reduction pathways in photosynthetic organisms and microorganisms. In vascular plant species, transport proteins and enzymes in this pathway are functionally diversified to have distinct biochemical properties in specific cellular and subcellular compartments. Recent findings indicate regulatory processes of sulfate transport and metabolism are tightly connected through several modes of transcriptional and posttranscriptional mechanisms. This review provides up-to-date knowledge in functions and regulations of sulfur assimilation in plants and algae, focusing on sulfate transport systems and metabolic pathways for sulfate reduction and synthesis of downstream metabolites with diverse biological functions.

Sulfur Assimilation in Photosynthetic Organisms: Molecular Functions and Regulations of Transporters and Assimilatory Enzymes

Acute adrenocorticotropic hormone stimulation of adrenal corticosteroidogenesis. Discovery of a rapidly induced protein.

Sulfite reductase (SiR) has been purified to homogeneity from spinach leaves. Two forms of the enzyme were separated by hydroxylapatite chromatography. One, with subunit Mr 69 000 appears to be proteolytically cleaved to give rise to the other, with subunit Mr 63 000, during the purification procedure. The two species have identical catalytic activities (on a per heme basis) when reduced methylviologen (MV+) or ferredoxin (Fdr) is used as electron donor for sulfite reduction, and they exhibit nearly identical optical and EPR spectra. Both enzyme forms exist in 50 mM phosphate buffer (pH 7.7) primarily as dimers at 20 degrees C. Spinach SiR contains 1 mol of siroheme and one Fe4S4 center per subunit. The heme iron is the high spin Fe3+ state in the enzyme as isolated. Near quantitative reduction of the Fe4S4 center by dithionite could be achieved if SiR was either converted to the CO complex or treated with 80% dimethyl sulfoxide. Spinach SiR and nitrite reductase (NiR) both catalyze Fdr-or MV+-de-pendent six-electron reductions of SO3(2)- and NO2-, as well as the two electron reduction of NH2OH. Vmax values are highest with the nitrogenous substrates. However, the Km of SiR for So3(2-), and of NiR for NO2-, is at least 2 orders of magnitude less than with either of the other substrates. Rates of reduction with Fdr as electron donor are greater than with MV+ as donor, No immunological cross-reaction could be detected between spinach SiR and Escherichia coli SiR or between spinach SiR and NiR.

Spinach siroheme enzymes: Isolation and characterization of ferredoxin-sulfite reductase and comparison of properties with ferredoxin-nitrite reductase.

Identification of the iron-sulfur center of spinach ferredoxin-nitrite reductase as a tetranuclear center, and preliminary EPR studies of mechanism.

https://www.jbc.org/content/257/11/6343.full.pdf

Rick J. Krueger

Papers

Acute adrenocorticotropic hormone stimulation of adrenal corticosteroidogenesis. Discovery of a rapidly induced protein.

Spinach siroheme enzymes: Isolation and characterization of ferredoxin-sulfite reductase and comparison of properties with ferredoxin-nitrite reductase.

Identification of the iron-sulfur center of spinach ferredoxin-nitrite reductase as a tetranuclear center, and preliminary EPR studies of mechanism.

Escherichia coli sulfite reductase hemoprotein subunit. Prosthetic groups, catalytic parameters, and ligand complexes.

Characterization of complexes between Escherichia coli sulfite reductase hemoprotein subunit and its substrates sulfite and nitrite.