Steroid Hormones and Nitrogen Excretion Metabolism

Mode of Action of Steroid Hormones

  • Steroid Hormones Definitions:

    • Aldosterone and cortisol are examples of steroid hormones.

    • These hormones affect the production of specific mRNA, which results in the production of specific proteins.

    • Implication: Steroids impact the processes of transcription and translation within the cell.

Steroid Hormone Mechanism

  • Overview of Action:

    • Steroids can easily pass through the cell membrane due to their lipophilic nature.

    • Once inside, they bind to intracellular receptors.

    • The hormone/receptor complex then moves into the nucleus.

    • In the nucleus, this complex interacts with specific DNA sequences.

  • Detailed Steps:

    1. Diffusion into Cell:

    • Free (unbound) steroid hormone diffuses through the plasma membrane.

    1. Binding to Cytosolic Receptors:

    • Once inside, steroid hormones bind to specific cytosolic receptors, which are also referred to as nuclear receptors.

    1. Hormone-Receptor Complex Formation:

    • The binding of the hormone-receptor complex occurs at the hormone response element (HRE) within the DNA.

    1. Transcription Activation:

    • This activation leads to the transcription of mRNA, resulting in the synthesis of new proteins that alter cell function.

Steroid Hormones and Gene Regulatory Proteins

  • Role of Gene Regulatory Proteins:

    • Steroid hormones like cortisol facilitate the activation of gene regulatory proteins that perform various regulatory functions within the cell.

    • The process involves a conformational change that activates the receptor protein, enabling it to function as a transcription factor that regulates gene expression.

Comparison of Steroid and Protein Hormones

  • Steroid Hormones:

    • Slower acting than protein hormones.

    • This is because they require the synthesis of new proteins, in contrast to protein hormones that primarily work through post-translational modifications, such as phosphorylation of existing proteins.

    • Their mechanism involves direct interaction with DNA and gene expression.

  • Protein Hormones:

    • Operate through second messenger systems, such as cAMP, which activate protein kinases.

    • This leads to phosphorylation of transcriptional activating factors, impacting the expression of specific genes.

Renin-Angiotensin-Aldosterone Pathway

  • Body’s Response to Decrease in Blood Pressure:

    • To understand this pathway, one should create a flow diagram starting with the sensory system detecting a drop in blood pressure.

Nitrogen Metabolism & Excretion

  • Relation to Water Balance:

    • The metabolism of nitrogen is closely related to the body's water balance, given that 90% of nitrogen comes from protein and amino acid breakdown.

  • Key Processes:

    1. De-amination:

    • An amino acid is metabolized, removing its amino group (NH2) to form ammonia (NH3), which converts to ammonium (NH4+).

    • Note: Ammonium can be toxic.

    1. Transamination:

    • An amino group is transferred from one amino acid to another, typically transferring to glutamate, which is then converted to glutamine.

Detailed Process Flow

  • Transdeamination Overview:

    • In the kidney or gills, glutamine undergoes deamination, liberating ammonia.

Chemical Reactions:

  1. Example of Transamination:

  • Alanine + α-ketoglutaric acid → Pyruvic acid & Glutamic acid.

  1. Conversion:

  • NH3 + Glutamic acid (using glutaminase enzyme) → Glutamine (with ATP → ADP).

    • These reactions involve glutamic acid and glutamine being transported in the blood to kidneys or gills.

Forms of Nitrogen Excretion

  • Types of Nitrogen Waste:

    1. Ammonia:

    2. Urea:

    3. Uric Acid:

  • The choice of nitrogenous waste form is influenced by the water balance situation of the animal and the embryonic development conditions.

Detailed Characteristics of Nitrogen Excretion Forms

  • Ammoniotelic Animals:

    • Typically excrete ammonia (NH3/NH4+), which is highly toxic but soluble in water.

    • Common in freshwater and marine invertebrates, as well as teleosts (bony fish).

  • Ureotelic Animals:

    • Excretion of urea, less toxic than ammonia, but requires energy for synthesis via the urea cycle.

    • Common in mammals, some fish, and amphibians transitioning from water to land.

    • The ornithine-urea cycle is a crucial metabolic pathway in this context.

  • Uricotelic Animals:

    • Rely on uric acid for excretion, which is relatively insoluble and therefore conserves water.

    • Common in terrestrial organisms such as birds, reptiles, and insects.

Nitrogen Excretion Mechanisms

  • Ammoniotelic Excretion:

    • Example: Aquatic invertebrates expel ammonia, which requires a significant amount of water.

  • Ureotelic Excretion:

    • Example: Mammals utilize urea with metabolic pathways like the Krebs-Ornithine cycle for synthesis, demanding energy input but being less toxic.

  • Uricotelic Excretion:

    • Advantages include minimized water loss due to uric acid's low solubility, suitable for arid environments.

Conclusion

  • The choice between ammonia, urea, or uric acid excretion strategies is largely dictated by water availability and the evolutionary adaptations of the organisms.