Synthesis and Degradation of Proteins in Pigs



Fig. 8.1
Insulin- and nutrition (main are amino acid and glucose)-signaling pathway leading to the stimulation of translation initiation. Activation of the insulin-signaling pathway is initiated by the binding of insulin to its receptor. This activates the insulin receptor and insulin receptor substrate-1 (IRS-1), followed by the activation of phosphoinositide-3 kinase (PI 3-K). Activated PI 3-K then stimulates the activation of phosphoinositide-dependent kinase 1 (PDK-1) and protein kinase B (PKB). Phosphorylation of PKB inactivates tuberous sclerosis complex 1/2 (TSC1/2), thereby inducing the activation of mammalian target of rapamycin (mTOR). Amino acids as well as insulin can activate mTOR, which exists in a complex with raptor and G protein β-subunit-like protein (GβL). Activated mTOR phosphorylates ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor (eIF) 4E-binding protein-1 (4EBP1). Phosphorylation of S6K1 enhances the activation of ribosomal subunit S6 (rpS6), which increases the translation of specific mRNA. Phosphorylated 4EBP1 releases eIF4E from an inactive eIF4E–4EBP1 complex, allowing the formation of the active eIF4G–eIF4E complex, which mediates the binding of mRNA to the 43S ribosomal complex. Insulin signaling can be attenuated by protein tyrosine phosphatase-1B (PTP-1B), which dephosphorylates the insulin receptor and IRS-1; phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which inactivates PI 3-K; and protein phosphatase 2A (PP2A), which acts on PKB and S6K1. An increase in adenosine monophosphate (AMP) levels enhances AMP kinase activation, resulting in the activation of TSC1/2 complex and the decreased activation of mTOR (Davis et al. 2008)





8.2.2 Feeding Induces Protein Syntheses in Pig


The feeding causes complex physiological and biochemical responses in animals. The circulating hormones (e.g., insulin) and nutritional metabolites (e.g., glucose and amino acids) are relatively elevated even with no food intake in pigs, and therefore ameliorate the whole-body nutritional metabolic status. Although with some differences in response to different dietary treatments (Deng et al. 2009; Deng et al. 2010), the postprandial circulating concentrations of glucose, amino acids, and insulin raised and are maintained at relatively high levels for 4 h after the meal and then returned to the baseline whatever pigs consumed their daily allowance under the feeding procedures of “six-time intake per day” (Yin et al. 2010; 2011; Deng et al. 2009; Deng et al. 2010), “three-time intake per day” (Liu et al. 2007), or “two-time intake per day” (Regmi et al. 2010) (Tables 8.1 and 8.2, Figs. 8.2 and 8.3). Therefore the whole-body fractional protein syntheses rate may be higher within the postprandial 4 h than during the time from postprandial 4th hour till next meal. In young pigs, the feeding stimulates protein synthesis in most tissues, and the efficiency by which dietary amino acids are used for protein deposition is high (Burrin et al. 1992). Notably, the most profound postprandial increase in protein synthesis occurs in skeletal muscle, which is of high metabolic significance, because skeletal muscle comprises a large proportion of the body mass in pigs. Feeding promotes the protein synthesis largely through glucose-, amino acid-, or insulin-induced formation of the active eukaryotic translation initiation factor (eIF) 4E.eIF4G complex or mTOR pathways (Kimball et al. 2000; Avruch et al. 2006; Jeyapalan et al. 2007; Suryawan et al. 2007). The phosphorylation of mTOR peaked within 30 min after starting the feeding, followed by the phosphorylation of the downstream targets of mTORC1, 4E-BP1, eIF4G, and S6 (Fig. 8.4), and then regulation of the binding of the mRNA with the 43S pre-initiation complex (Wilson et al 2009). These physiological and biochemical processes paralleled the stimulation of skeletal muscle protein synthesis and led to the enhancement of protein synthesis in pigs. In addition, the rate of translation initiation is up-regulated earlier than elongation after feeding. Although the elongation is also increased after meal consumption, this increase is delayed relative to the change in initiation. Interestingly, increased rates of elongation did not further increase muscle protein synthesis in response to feeding, which therefore indicates that initiation is the rate-limiting step in translation (Wilson et al. 2009). Therefore, the increased rates of initiation, but not elongation, are responsible for the increase in muscle protein synthesis in response to feeding in pigs.


Table 8.1
Serum amino acid concentrations after first meal in response to different dietary treatment in pigs (mmol/L)















































































































































































































































































































































Item

Sampling time

Pooled SEM

Time effect, P value

8:30

9:50

10:50

11:50

HDRS

MDRS

LDRS

HDRS

MDRS

LDRS

HDRS

MDRS

LDRS

HDRS

MDRS

LDRS

Nutritionally indispensable amino acids

Arginine

0.089

0.087

0.083

0.179a

0.128b

0.094b

0.163a

0.107b

0.084c

0.070

0.068

0.064

0.004

<0.001

Cysteine

0.055a

0.045a,b

0.038b

0.086a

0.064b

0.068b

0.059

0.045

0.046

0.037

0.034

0.034

0.001

<0.001

Histidine

0.038

0.036

0.032

0.062a

0.041b

0.045b

0.038

0.032

0.044

0.032

0.028

0.040

0.001

0.018

Isoleucine

0.050

0.047

0.048

0.090a

0.055b

0.060b

0.061a

0.050a,b

0.048b

0.049a

0.034b

0.038a,b

0.001

0.001

Leucine

0.205a

0.215a

0.173b

0.310a

0.260b

0.245b

0.271a

0.254a,b

0.204b

0.148

0.154

0.142

0.004

<0.001

Lysine

0.287a

0.261a,b

0.233b

0.583a

0.574a

0.297b

0.320a

0.233b

0.227b

0.242a

0.220a

0.158b

0.009

0.004

Methionine

0.037

0.031

0.031

0.055a

0.038b

0.033b

0.061a

0.034b

0.036b

0.040a

0.029b

0.028b

0.002

0.004

Phenylalanine

0.085

0.089

0.083

0.137a

0.116b

0.102b

0.120a

0.100b

0.103b

0.082

0.071

0.074

0.002

0.001

Threonine

0.452a

0.380b

0.375b

0.525a

0.422c

0.460b

0.384a

0.342a,b

0.308b

0.285a

0.294a

0.247b

0.010

<0.001

Tryptophan

0.235a

0.197b

0.183b

0.288a

0.237b

0.223b

0.246a

0.213b

0.200b

0.200a

0.166b

0.143b

0.007

0.009

Tyrosine

0.104a

0.093a,b

0.084b

0.164a

0.118b

0.117b

0.152a

0.114b

0.093b

0.101a

0.076a,b

0.068b

0.002

<0.001

Valine

0.112

0.094

0.100

0.166a

0.098c

0.129b

0.138a

0.101b

0.100b

0.121a

0.074b

0.087b

0.002

0.011

Nutritionally dispensable amino acids

Alanine

0.387a

0.334b

0.350a,b

0.450a

0.389b

0.384b

0.476a

0.465a,b

0.414b

0.406a

0.390a

0.366b

0.011

0.001

Aspartate

0.025

0.022

0.028

0.030

0.032

0.030

0.049

0.042

0.042

0.034a

0.026b

0.022b

0.001

0.001

Glutamate

0.241

0.227

0.226

0.248b

0.278a,b

0.298a

0.324a

0.318a,b

0.279b

0.310a

0.205c

0.260b

0.007

0.010

Glycine

0.463a

0.432a,b

0.410b

0.541a

0.520a,b

0.487b

0.549a

0.545a,b

0.522b

0.471a

0.473a

0.427b

0.015

0.014

Proline

0.337a

0.359a

0.256b

0.623a

0.577b

0.555b

0.541a

0.492b

0.435c

0.205

0.232

0.233

0.012

<0.001

Serine

0.105a

0.095a,b

0.084b

0.117

0.124

0.122

0.089a,b

0.095a

0.080b

0.088

0.084

0.071

0.001

<0.001


a–cWithin the same sampling time, values in a row sharing different superscript letters differ (P < 0.05); n = 6

HDRS-fed group high digestion rate starch-fed group, LDRS low digestion rate starch-fed group, MDRS moderate digestion rate starch, SEM standard error of the mean

Source: Yin et al. (2010) (with minor modification, the pigs were fed at 8:00 am)



Table 8.2
Serum amino acid concentrations after second feeding in response to different dietary treatment in pigs (mmol/L, continued from Table 8.1)








































































































Item

Sampling time

Pooled SEM

Time effect, P value

12:30

13:30

14:30

15:30

HDRS

MDRS

LDRS

HDRS

MDRS

LDRS

HDRS

MDRS

LDRS

HDRS

MDRS

LDRS

Nutritionally indispensable amino acids

Arginine

0.112a

0.079b

0.100a,b

0.181a

0.132b

0.128b

0.141a

0.115a,b

0.080b

0.074

0.065

0.069

0.004

<0.001

Cysteine

0.036a

0.029b

0.036a

0.056a

0.043b

0.041b

0.037

0.034

0.034

0.034

0.033

0.030

0.001

<0.001

Histidine

0.051

0.043

0.046

0.063a

0.041b

0.049b

0.041

0.042

0.050

0.037

0.031

0.040

0.001

0.018

Isoleucine

0.057a

0.037b

0.056a

0.090a

0.055b

0.060b

0.066

0.056

0.047

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Apr 21, 2017 | Posted by in GENERAL | Comments Off on Synthesis and Degradation of Proteins in Pigs

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